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/models.py

@@ -146,6 +146,10 @@ class Video(Base):
     is_short: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false", index=True
     )
+    # Whether the youtube.com/shorts/<id> probe has run for this video.
+    shorts_probed: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false", index=True
+    )
     # none | live | upcoming | premiere | was_live
     live_status: Mapped[str] = mapped_column(
         String(16), default="none", server_default="none", index=True