fix: address reader-UI feedback (shorts, search, tags, login, UX)

- Shorts: confirm via youtube.com/shorts/<id> probe (SOCS cookie bypasses the
  consent redirect) instead of a 60s heuristic; concurrent probing, shorts_probed
  flag, scheduled refinement (migration 0005)
- Search: match title + channel name only (descriptions caused noisy results)
- Faceted tag filtering: AND across categories (language AND topic narrows),
  OR within a category; any/all toggle applies to topics
- Language detection: majority vote over individual titles (fixes misdetections
  like multipoleguy -> English; drops bogus Polish/Romanian)
- Login: drop forced consent so returning sign-in is quick (select_account)
- Feed cards: clickable channel name (opens channel), persistent saved badge,
  undo toast on hide, Hidden view to restore; tag chips show counts in tooltip

backend/app/youtube/shorts.py

@@ -0,0 +1,32 @@
+"""Confirm whether a video is a Short by probing youtube.com/shorts/<id>.
+
+A real Short returns HTTP 200 at that URL; a regular video redirects to /watch.
+This uses no API quota."""
+import httpx
+
+SHORTS_URL = "https://www.youtube.com/shorts/{video_id}"
+# The SOCS cookie skips YouTube's cookie-consent interstitial, which otherwise
+# redirects every server-side request to consent.youtube.com.
+_HEADERS = {
+    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64)",
+    "Cookie": "SOCS=CAI",
+}
+
+
+def probe_is_short(client: httpx.Client, video_id: str) -> bool | None:
+    """True if a Short, False if a regular video, None if undetermined (retry later)."""
+    try:
+        resp = client.get(
+            SHORTS_URL.format(video_id=video_id), follow_redirects=False
+        )
+    except httpx.HTTPError:
+        return None
+    if resp.status_code == 200:
+        return True
+    if resp.status_code in (301, 302, 303, 307, 308):
+        return False
+    return None
+
+
+def make_client() -> httpx.Client:
+    return httpx.Client(timeout=10.0, headers=_HEADERS)