Merge feature/s3-dynamic-facets: dynamic faceted filter chips

S3a:
- feat(feed): /api/facets endpoint for contextual per-tag channel counts
- feat(filters): dynamic chips (live counts, hide non-matching, sort by count)
- feat(filters): prominent topic Any/All match toggle
- fix(feed): conjunctive facet counts in AND mode

backend/app/routes/feed.py

@@ -67,6 +67,7 @@ def _filtered_query(
     include_live: bool,
     show: str,
     scope: str = "my",
+    exclude_tag_category: str | None = None,
 ) -> tuple[Select, object]:
     """Build the feed query (joins + all WHERE filters), shared by /feed and /feed/count.
     Returns the column-bearing select plus the watch-status expression for sorting.
@@ -153,6 +154,11 @@ def _filtered_query(
         by_category: dict[str, list[int]] = {}
         for tag_id, category in cat_rows:
             by_category.setdefault(category, []).append(tag_id)
+        # Facet counting drops the category being counted so its own chips don't zero each
+        # other out (standard drill-down faceting: a category's count ignores its own
+        # selections but still honours the other categories' filters).
+        if exclude_tag_category:
+            by_category.pop(exclude_tag_category, None)
         visible = or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
         for category, ids in by_category.items():
             if category == "topic" and tag_mode == "and" and len(ids) > 1:
@@ -294,6 +300,48 @@ def get_feed_count(
     return {"count": total or 0}
 
 
+@router.get("/facets")
+def get_facets(
+    params: dict = Depends(_feed_params),
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Per-tag channel counts for the *current* filter context, so the sidebar can show
+    live counts and drop chips that no longer match anything. Each category is counted with
+    every other filter applied (scope, channel, date, content type, search, watch state,
+    and the other category's tags) but its own selections ignored — standard drill-down
+    faceting, so selecting one topic doesn't zero out the rest of the topics.
+
+    The count is the number of distinct channels that have at least one video in the current
+    view, matching the existing channel-count chip semantics. JSON object keys are strings
+    (tag ids)."""
+    visible = or_(ChannelTag.user_id.is_(None), ChannelTag.user_id == user.id)
+    counts: dict[int, int] = {}
+    for category in ("language", "topic"):
+        # Disjunctive (OR) facets drop the category's own selections so its chips keep
+        # independent counts (you can OR more of them in). Conjunctive (AND, topics only)
+        # keeps them applied, so each remaining chip narrows to channels that ALSO have all
+        # already-selected topics — and tags that can't co-occur drop to zero (hidden).
+        conjunctive = category == "topic" and params.get("tag_mode") == "and"
+        base, _status = _filtered_query(
+            db,
+            user,
+            **{**params, "exclude_tag_category": None if conjunctive else category},
+        )
+        channels = base.with_only_columns(Video.channel_id).distinct().subquery()
+        rows = db.execute(
+            select(ChannelTag.tag_id, func.count(func.distinct(ChannelTag.channel_id)))
+            .select_from(channels)
+            .join(ChannelTag, ChannelTag.channel_id == channels.c.channel_id)
+            .join(Tag, and_(Tag.id == ChannelTag.tag_id, Tag.category == category))
+            .where(visible)
+            .group_by(ChannelTag.tag_id)
+        ).all()
+        for tag_id, count in rows:
+            counts[tag_id] = count
+    return {"counts": counts}
+
+
 @router.post("/videos/{video_id}/state")
 def set_video_state(
     video_id: str,

frontend/src/components/Sidebar.tsx

@@ -74,10 +74,12 @@ const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q" | "scope"> = {
 
 function TagChip({
   tag,
+  count,
   active,
   onClick,
 }: {
   tag: Tag;
+  count: number;
   active: boolean;
   onClick: () => void;
 }) {
@@ -85,7 +87,7 @@ function TagChip({
   return (
     <button
       onClick={onClick}
-      title={t("sidebar.channelCount", { count: tag.channel_count })}
+      title={t("sidebar.channelCount", { count })}
       className={`inline-flex items-center gap-1.5 text-xs px-2.5 py-1 rounded-full border shadow-sm hover:shadow active:translate-y-px transition ${
         active
           ? "bg-accent text-accent-fg border-accent"
@@ -98,7 +100,7 @@ function TagChip({
           active ? "bg-accent-fg/20 text-accent-fg" : "bg-muted/15 text-muted"
         }`}
       >
-        {tag.channel_count}
+        {count}
       </span>
     </button>
   );
@@ -119,6 +121,31 @@ export default function Sidebar({
   const tagsQuery = useQuery({ queryKey: ["tags"], queryFn: api.tags });
   const tags = tagsQuery.data ?? [];
 
+  // Live per-tag channel counts for the current filter context (scope, channel, date,
+  // search, watch state, other category's tags). Lets us show contextual counts and hide
+  // chips that no longer match anything. Keyed on filters so it refetches as they change.
+  const facetsQuery = useQuery({
+    queryKey: ["facets", filters],
+    queryFn: () => api.facets(filters),
+    staleTime: 30_000,
+  });
+  const facetsReady = !!facetsQuery.data;
+  const facetCounts = facetsQuery.data?.counts ?? {};
+  // Before facets load, fall back to the static global count so chips don't flash empty.
+  const chipCount = (tag: Tag): number =>
+    facetsReady ? facetCounts[String(tag.id)] ?? 0 : tag.channel_count;
+  // Visible chips: hide zero-count ones once facets are in, but always keep selected ones
+  // so an active filter can still be cleared. Sorted by count (largest first), then name —
+  // so scanning top→bottom the smallest counts end up at the very bottom.
+  const visibleChips = (list: Tag[]): Tag[] => {
+    const shown = facetsReady
+      ? list.filter((tg) => chipCount(tg) > 0 || filters.tags.includes(tg.id))
+      : list;
+    return [...shown].sort(
+      (a, b) => chipCount(b) - chipCount(a) || a.name.localeCompare(b.name)
+    );
+  };
+
   // After a page refresh the channel name isn't in the URL (only the id is), so
   // filters.channelName is undefined and the chip would fall back to "This channel".
   // Resolve the title from the (cached) channel list keyed by id. Only fetch when a
@@ -362,45 +389,74 @@ export default function Sidebar({
             />
           </>
         );
-      case "language":
+      case "language": {
+        const chips = visibleChips(languages);
         return (
           <div className="flex flex-wrap gap-1.5">
-            {languages.map((t) => (
+            {chips.map((tg) => (
               <TagChip
-                key={t.id}
-                tag={t}
-                active={filters.tags.includes(t.id)}
-                onClick={() => toggleTag(t.id)}
+                key={tg.id}
+                tag={tg}
+                count={chipCount(tg)}
+                active={filters.tags.includes(tg.id)}
+                onClick={() => toggleTag(tg.id)}
               />
             ))}
+            {facetsReady && chips.length === 0 && (
+              <span className="text-[11px] text-muted">{t("sidebar.noMatchingTags")}</span>
+            )}
           </div>
         );
-      case "topic":
+      }
+      case "topic": {
+        const chips = visibleChips(topics);
         return (
           <>
-            <div className="flex justify-end mb-1.5">
-              <button
-                onClick={() =>
-                  setFilters({ ...filters, tagMode: filters.tagMode === "or" ? "and" : "or" })
-                }
-                className="text-[10px] uppercase tracking-wide text-muted hover:text-accent"
-                title={t("sidebar.tagModeTooltip")}
+            <div
+              className="flex items-center justify-between mb-1.5"
+              title={t("sidebar.tagModeTooltip")}
+            >
+              <span className="text-[10px] uppercase tracking-wide text-muted">
+                {t("sidebar.match")}
+              </span>
+              <div
+                className="flex items-center rounded-full border border-border bg-card p-0.5 text-[10px]"
+                role="group"
+                aria-label={t("sidebar.match")}
               >
-                {filters.tagMode === "or" ? t("sidebar.any") : t("sidebar.all")}
-              </button>
+                {(["or", "and"] as const).map((m) => (
+                  <button
+                    key={m}
+                    onClick={() => setFilters({ ...filters, tagMode: m })}
+                    aria-pressed={filters.tagMode === m}
+                    className={`px-2 py-0.5 rounded-full transition ${
+                      filters.tagMode === m
+                        ? "bg-accent text-accent-fg"
+                        : "text-muted hover:text-fg"
+                    }`}
+                  >
+                    {m === "or" ? t("sidebar.any") : t("sidebar.all")}
+                  </button>
+                ))}
+              </div>
             </div>
             <div className="flex flex-wrap gap-1.5">
-              {topics.map((t) => (
+              {chips.map((tg) => (
                 <TagChip
-                  key={t.id}
-                  tag={t}
-                  active={filters.tags.includes(t.id)}
-                  onClick={() => toggleTag(t.id)}
+                  key={tg.id}
+                  tag={tg}
+                  count={chipCount(tg)}
+                  active={filters.tags.includes(tg.id)}
+                  onClick={() => toggleTag(tg.id)}
                 />
               ))}
+              {facetsReady && chips.length === 0 && (
+                <span className="text-[11px] text-muted">{t("sidebar.noMatchingTags")}</span>
+              )}
             </div>
           </>
         );
+      }
     }
   }
 

frontend/src/i18n/locales/de/sidebar.json

@@ -19,11 +19,13 @@
   "any": "Beliebig",
   "all": "Alle",
   "tagModeTooltip": "Beliebige vs. alle ausgewählten Tags treffen",
+  "match": "Treffer",
   "custom": "Benutzerdefiniert",
   "from": "Von",
   "to": "Bis",
   "clearDates": "Daten löschen",
   "reshuffle": "Neu mischen",
+  "noMatchingTags": "Keine passenden Tags",
   "widget": {
     "show": "Anzeigen",
     "sort": "Sortierung",

frontend/src/i18n/locales/en/sidebar.json

@@ -19,11 +19,13 @@
   "any": "Any",
   "all": "All",
   "tagModeTooltip": "Match any vs all selected tags",
+  "match": "Match",
   "custom": "Custom",
   "from": "From",
   "to": "To",
   "clearDates": "clear dates",
   "reshuffle": "Reshuffle",
+  "noMatchingTags": "No matching tags here",
   "widget": {
     "show": "Show",
     "sort": "Sort",

frontend/src/i18n/locales/hu/sidebar.json

@@ -19,11 +19,13 @@
   "any": "Bármelyik",
   "all": "Összes",
   "tagModeTooltip": "Bármelyik vagy az összes kijelölt címke illeszkedjen",
+  "match": "Egyezés",
   "custom": "Egyéni",
   "from": "Ettől",
   "to": "Eddig",
   "clearDates": "dátumok törlése",
   "reshuffle": "Újrakeverés",
+  "noMatchingTags": "Nincs ide illő címke",
   "widget": {
     "show": "Megjelenítés",
     "sort": "Rendezés",

frontend/src/lib/api.ts

@@ -152,13 +152,13 @@ async function req(url: string, opts: RequestInit = {}): Promise<any> {
   return r.status === 204 ? null : r.json();
 }
 
-function feedQuery(f: FeedFilters, offset: number, limit: number): string {
+// The filter half of the query (everything except paging/sort), shared by the feed and
+// the facet-count endpoint so both see exactly the same filter context.
+function filterParams(f: FeedFilters): URLSearchParams {
   const p = new URLSearchParams();
   f.tags.forEach((t) => p.append("tags", String(t)));
   p.set("tag_mode", f.tagMode);
   if (f.q) p.set("q", f.q);
-  p.set("sort", f.sort);
-  if (f.sort === "shuffle" && f.seed) p.set("seed", String(f.seed));
   p.set("scope", f.scope);
   p.set("show_normal", String(f.includeNormal));
   p.set("include_shorts", String(f.includeShorts));
@@ -170,6 +170,13 @@ function feedQuery(f: FeedFilters, offset: number, limit: number): string {
   if (f.dateTo) p.set("published_before", f.dateTo);
   if (f.minDuration != null) p.set("min_duration", String(f.minDuration));
   if (f.maxDuration != null) p.set("max_duration", String(f.maxDuration));
+  return p;
+}
+
+function feedQuery(f: FeedFilters, offset: number, limit: number): string {
+  const p = filterParams(f);
+  p.set("sort", f.sort);
+  if (f.sort === "shuffle" && f.seed) p.set("seed", String(f.seed));
   p.set("offset", String(offset));
   p.set("limit", String(limit));
   return p.toString();
@@ -252,6 +259,8 @@ export const api = {
     req(`/api/feed?${feedQuery(f, offset, limit)}`),
   feedCount: (f: FeedFilters): Promise<{ count: number }> =>
     req(`/api/feed/count?${feedQuery(f, 0, 0)}`),
+  facets: (f: FeedFilters): Promise<{ counts: Record<string, number> }> =>
+    req(`/api/facets?${filterParams(f).toString()}`),
   setState: (id: string, status: string) =>
     req(`/api/videos/${id}/state`, { method: "POST", body: JSON.stringify({ status }) }),
   saveProgress: (id: string, positionSeconds: number, durationSeconds: number) =>