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/feed.py

@@ -14,6 +14,7 @@ from app import quota
 from app.auth import current_user
 from app.db import get_db
 from app.models import (
+    LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     ChannelTag,
@@ -34,7 +35,6 @@ router = APIRouter(prefix="/api", tags=["feed"])
 
 # "saved" used to be a status; it's now membership in the built-in Watch later playlist.
 VALID_STATES = {"new", "watched", "hidden"}
-HIDDEN_LIVE = ("live", "upcoming")
 
 # Resume-position thresholds (mirror the client): positions below this are "didn't
 # really start" and within this of the end are "basically finished" — neither is worth
@@ -132,7 +132,7 @@ def _filtered_query(
     exclude_tag_category: str | None = None,
 ) -> tuple[Select, object]:
     """Build the feed query (joins + all WHERE filters), shared by /feed and /feed/count.
-    Returns the column-bearing select plus the watch-status expression for sorting.
+    Returns the column-bearing select plus the priority/relevance rank expression for sorting.
 
     `scope="my"` (default) restricts the feed to the user's own non-hidden subscriptions.
     `scope="all"` shows every video in the shared catalog (any user's ingested channels);
@@ -313,12 +313,12 @@ def _filtered_query(
         type_clauses = []
         if show_normal:
             type_clauses.append(
-                and_(Video.is_short.is_(False), Video.live_status.notin_(HIDDEN_LIVE))
+                and_(Video.is_short.is_(False), Video.live_status.notin_(LIVE_OR_UPCOMING))
             )
         if include_shorts:
             type_clauses.append(Video.is_short.is_(True))
         if include_live:
-            type_clauses.append(Video.live_status.in_(HIDDEN_LIVE))
+            type_clauses.append(Video.live_status.in_(LIVE_OR_UPCOMING))
         query = query.where(or_(*type_clauses) if type_clauses else false())
 
     if show == "unwatched":
@@ -335,7 +335,7 @@ def _filtered_query(
     else:  # all
         query = query.where(status_expr != "hidden")
 
-    return query, status_expr, rank_expr
+    return query, rank_expr
 
 
 def _to_tsquery_str(q: str) -> str | None:
@@ -492,7 +492,7 @@ def get_feed(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    query, _status, rank_expr = _filtered_query(db, user, **params)
+    query, rank_expr = _filtered_query(db, user, **params)
     keys = _sort_keys(sort, seed, rank_expr)
 
     # Expose each key column on the row so we can build the next cursor from the last item.
@@ -522,7 +522,7 @@ def get_feed_count(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    query, _status, _rank = _filtered_query(db, user, **params)
+    query, _rank = _filtered_query(db, user, **params)
     total = db.scalar(select(func.count()).select_from(query.subquery()))
     return {"count": total or 0}
 
@@ -550,7 +550,7 @@ def get_facets(
         # keeps them applied, so each remaining chip narrows to channels that ALSO have all
         # already-selected topics — and tags that can't co-occur drop to zero (hidden).
         conjunctive = category == "topic" and params.get("tag_mode") == "and"
-        base, _status, _rank = _filtered_query(
+        base, _rank = _filtered_query(
             db,
             user,
             **{**params, "exclude_tag_category": None if conjunctive else category},