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/routes/channels.py

@@ -12,6 +12,7 @@ from app import quota, sysconfig
 from app.auth import current_user, has_write_scope, require_human
 from app.db import get_db
 from app.models import (
+    LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     ChannelTag,
@@ -59,7 +60,7 @@ def list_channels(
                 func.max(Video.published_at),
                 func.coalesce(func.sum(Video.duration_seconds), 0),
                 func.count(Video.id).filter(Video.is_short.is_(True)),
-                func.count(Video.id).filter(Video.live_status.in_(("live", "upcoming"))),
+                func.count(Video.id).filter(Video.live_status.in_(LIVE_OR_UPCOMING)),
             )
             .where(Video.channel_id.in_(channel_ids))
             .group_by(Video.channel_id)