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/youtube/client.py

@@ -167,9 +167,9 @@ class YouTubeClient:
             if not page_token:
                 break
 
-    def iter_my_playlist_video_ids(self, playlist_id: str) -> Iterator[str]:
-        """Yield the video ids of one of the user's own playlists, in playlist order
-        (OAuth, so private/unlisted playlists work)."""
+    def _iter_playlist_items(self, playlist_id: str) -> Iterator[dict]:
+        """Paginate one of the user's playlists' items (playlistItems, contentDetails part;
+        OAuth so private/unlisted work), yielding each raw item in playlist order."""
         page_token = None
         while True:
             params = {
@@ -180,14 +180,19 @@ class YouTubeClient:
             if page_token:
                 params["pageToken"] = page_token
             data = self._get("playlistItems", params, cost=1, allow_key=False)
-            for item in data.get("items", []):
-                vid = item.get("contentDetails", {}).get("videoId")
-                if vid:
-                    yield vid
+            yield from data.get("items", [])
             page_token = data.get("nextPageToken")
             if not page_token:
                 break
 
+    def iter_my_playlist_video_ids(self, playlist_id: str) -> Iterator[str]:
+        """Yield the video ids of one of the user's own playlists, in playlist order
+        (OAuth, so private/unlisted playlists work)."""
+        for item in self._iter_playlist_items(playlist_id):
+            vid = item.get("contentDetails", {}).get("videoId")
+            if vid:
+                yield vid
+
     def get_my_channel_id(self) -> str | None:
         """The authenticated user's own channel id (channels.list?mine=true). OAuth only;
         1 quota unit. Returns None if the account has no channel."""
@@ -264,23 +269,10 @@ class YouTubeClient:
         """Yield {item_id, video_id} for each item of one of the user's playlists, in
         playlist order. `item_id` is the playlistItem resource id, needed to delete or
         reposition the item via the write API. OAuth (private playlists work)."""
-        page_token = None
-        while True:
-            params = {
-                "part": "contentDetails",
-                "playlistId": playlist_id,
-                "maxResults": 50,
-            }
-            if page_token:
-                params["pageToken"] = page_token
-            data = self._get("playlistItems", params, cost=1, allow_key=False)
-            for item in data.get("items", []):
-                vid = item.get("contentDetails", {}).get("videoId")
-                if vid:
-                    yield {"item_id": item.get("id"), "video_id": vid}
-            page_token = data.get("nextPageToken")
-            if not page_token:
-                break
+        for item in self._iter_playlist_items(playlist_id):
+            vid = item.get("contentDetails", {}).get("videoId")
+            if vid:
+                yield {"item_id": item.get("id"), "video_id": vid}
 
     # --- write endpoints (OAuth only, never the API key; each costs 50 quota units) ---
     def _write(self, method: str, path: str, *, params: dict, json: dict | None = None) -> dict: