feat(channels): status filter + header full-history link + stable priority

- header: per-user "N without full history" count (channels_deep_pending),
  clickable with a hint -> opens the channel manager filtered to those.
- channel manager: status filter chips (All / Needs full history / Fully synced
  / Hidden); the header link deep-links to "Needs full history".
- fix: priority up/down is now an optimistic in-place cache update (no refetch /
  re-sort), so the list no longer jumps to the top and loses your scroll position;
  the new order applies on the next page load.

frontend/src/components/Channels.tsx

@@ -18,12 +18,25 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import Avatar from "./Avatar";
 
+export type ChannelStatusFilter = "all" | "needs_full" | "fully_synced" | "hidden";
+
+const STATUS_FILTERS: { id: ChannelStatusFilter; label: string }[] = [
+  { id: "all", label: "All" },
+  { id: "needs_full", label: "Needs full history" },
+  { id: "fully_synced", label: "Fully synced" },
+  { id: "hidden", label: "Hidden" },
+];
+
 export default function Channels({
   canWrite,
   onViewChannel,
+  statusFilter,
+  setStatusFilter,
 }: {
   canWrite: boolean;
   onViewChannel: (id: string, name: string) => void;
+  statusFilter: ChannelStatusFilter;
+  setStatusFilter: (f: ChannelStatusFilter) => void;
 }) {
   const qc = useQueryClient();
   const channelsQuery = useQuery({ queryKey: ["channels"], queryFn: api.channels });
@@ -54,6 +67,19 @@ export default function Channels({
       qc.invalidateQueries({ queryKey: ["feed-count"] });
     },
   });
+  // Priority is updated optimistically in place and NOT re-sorted/refetched, so the list
+  // doesn't jump (the server list is priority-sorted; refetching would reorder rows and
+  // lose your scroll position). The new order takes effect next time the page loads.
+  const bumpPriority = (id: string, current: number, delta: number) => {
+    const next = current + delta;
+    qc.setQueryData<ManagedChannel[]>(["channels"], (old) =>
+      (old ?? []).map((ch) => (ch.id === id ? { ...ch, priority: next } : ch))
+    );
+    api
+      .updateChannel(id, { priority: next })
+      .catch(() => qc.invalidateQueries({ queryKey: ["channels"] }));
+  };
+
   const attach = useMutation({
     mutationFn: (v: { id: string; tagId: number }) => api.attachChannelTag(v.id, v.tagId),
     onSuccess: () => qc.invalidateQueries({ queryKey: ["channels"] }),
@@ -103,9 +129,17 @@ export default function Channels({
     onError: () => notify({ level: "error", message: "Couldn't request full history" }),
   });
 
-  const channels = (channelsQuery.data ?? []).filter(
-    (c) => !q || (c.title ?? "").toLowerCase().includes(q.toLowerCase())
-  );
+  const channels = (channelsQuery.data ?? [])
+    .filter((c) => !q || (c.title ?? "").toLowerCase().includes(q.toLowerCase()))
+    .filter((c) =>
+      statusFilter === "needs_full"
+        ? !c.backfill_done
+        : statusFilter === "fully_synced"
+          ? c.backfill_done
+          : statusFilter === "hidden"
+            ? c.hidden
+            : true
+    );
   const s = statusQuery.data;
 
   return (
@@ -179,6 +213,23 @@ export default function Channels({
         </Tooltip>
       </div>
 
+      {/* Status filter */}
+      <div className="flex flex-wrap items-center gap-1.5 mb-4">
+        {STATUS_FILTERS.map((f) => (
+          <button
+            key={f.id}
+            onClick={() => setStatusFilter(f.id)}
+            className={`text-xs px-2.5 py-1 rounded-full border transition ${
+              statusFilter === f.id
+                ? "bg-accent text-accent-fg border-accent"
+                : "bg-card border-border text-muted hover:border-accent"
+            }`}
+          >
+            {f.label}
+          </button>
+        ))}
+      </div>
+
       <p className="text-xs text-muted mb-4 leading-relaxed">
         Set a channel's <b className="text-fg/80">priority</b> to push its videos up when you sort
         by “Channel priority”, attach your own <b className="text-fg/80">tags</b> to filter the feed,
@@ -244,7 +295,7 @@ export default function Channels({
                   unsubscribe.mutate(c.id);
               }}
               onView={() => onViewChannel(c.id, c.title ?? "This channel")}
-              onPriority={(d) => patch.mutate({ id: c.id, body: { priority: c.priority + d } })}
+              onPriority={(d) => bumpPriority(c.id, c.priority, d)}
               onHide={() => patch.mutate({ id: c.id, body: { hidden: !c.hidden } })}
               onDeep={() =>
                 patch.mutate({ id: c.id, body: { deep_requested: !c.deep_requested } })