chore(feed): Phase 2 #2 backend cleanup — dead return, dup helpers, shared const

- feed.py _filtered_query: drop the dead 2nd return element (status_expr was
  used only internally for WHERE filters; all 3 callers discarded it as _status).
  Now returns (query, rank_expr); fixed the stale docstring.
- youtube/client.py: extract _iter_playlist_items() — iter_my_playlist_video_ids
  and iter_playlist_items_with_ids were near-identical playlistItems paging loops
  (jscpd [173-187]≈[267-281]); both now map over the shared generator.
- sync/videos.py: extract _apply_video_batch() with an on_missing callback —
  enrich_pending and refresh_live shared the fetch-map-apply skeleton, differing
  only in the query and how they retire rows YouTube omits.
- models.py: add LIVE_OR_UPCOMING = ("live","upcoming"); replace the 4 duplicated
  copies (feed.HIDDEN_LIVE, search._LIVE_HIDDEN, videos.py + channels.py inline).

Behavior-neutral. ruff clean on touched files, localdev boots, feed/worker healthy.

backend/app/models.py

@@ -232,6 +232,11 @@ class Subscription(Base, TimestampMixin):
     channel: Mapped["Channel"] = relationship(back_populates="subscriptions")
 
 
+# Video.live_status values that mean "currently a live/upcoming broadcast" — transient states
+# hidden from feeds/search and revisited by the refresh job until they settle into a VOD.
+LIVE_OR_UPCOMING = ("live", "upcoming")
+
+
 class Video(Base, TimestampMixin):
     """A YouTube video. Shared across users; per-user state lives elsewhere."""