feat(playlists): YouTube playlist read-sync (backend)

Mirror each user's own YouTube playlists into local source='youtube' playlists,
one-way (YT -> local). New client methods iter_my_playlists / iter_my_playlist_video_ids
(OAuth, so private playlists work); sync/playlists.py reconciles the mirror (matching YT
order) and ingests any playlist videos not in the shared catalog yet (with stub channels).
POST /api/playlists/sync-youtube for manual sync (read-scope gated, per-user quota) plus a
scheduler job (playlist_sync_minutes, default 6h) that syncs all read-scope users. YouTube's
Watch Later / History are not API-accessible and are never synced.

backend/app/youtube/client.py

@@ -125,6 +125,49 @@ class YouTubeClient:
             if not page_token:
                 break
 
+    def iter_my_playlists(self) -> Iterator[dict]:
+        """Yield the authenticated user's own playlists (OAuth, so private ones are
+        included). Note: YouTube's special Watch Later / History playlists are NOT
+        returned by the Data API and cannot be synced."""
+        page_token = None
+        while True:
+            params = {"part": "snippet,contentDetails", "mine": "true", "maxResults": 50}
+            if page_token:
+                params["pageToken"] = page_token
+            data = self._get("playlists", params, cost=1, allow_key=False)
+            for item in data.get("items", []):
+                sn = item.get("snippet", {})
+                yield {
+                    "id": item.get("id"),
+                    "title": sn.get("title"),
+                    "item_count": item.get("contentDetails", {}).get("itemCount"),
+                    "thumbnail_url": best_thumbnail(sn.get("thumbnails")),
+                }
+            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)."""
+        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 vid
+            page_token = data.get("nextPageToken")
+            if not page_token:
+                break
+
     def get_channels(self, channel_ids: list[str]) -> list[dict]:
         items: list[dict] = []
         for batch in _chunks(channel_ids, 50):