feat: M2 (part 2) — RSS poller, backfill, enrichment, scheduler

- Free per-channel RSS reader for quota-less fresh-video detection
- Recent-first backfill (configurable: 100 videos / 1 year) plus resumable deep
  backfill from the uploads playlist
- Enrichment via videos.list: duration, view/like counts, category, topics,
  language, Shorts heuristic and livestream/premiere classification
- Reusable sync runners + APScheduler jobs (rss / enrich / backfill), all
  quota-aware with a reserve so backfill never starves fresh enrichment
- Manual triggers: POST /api/sync/{rss,backfill,enrich}
- Exact insert counting via RETURNING with in-batch de-duplication

backend/app/youtube/client.py

@@ -133,3 +133,29 @@ class YouTubeClient:
             )
             items.extend(data.get("items", []))
         return items
+
+    def get_playlist_items(self, playlist_id: str, page_token: str | None = None) -> dict:
+        """One page (up to 50) of a playlist's items, most-recent-first for uploads
+        playlists. Returns the raw response (items + nextPageToken)."""
+        params = {
+            "part": "contentDetails,snippet",
+            "playlistId": playlist_id,
+            "maxResults": 50,
+        }
+        if page_token:
+            params["pageToken"] = page_token
+        return self._get("playlistItems", params)
+
+    def get_videos(self, video_ids: list[str]) -> list[dict]:
+        items: list[dict] = []
+        for batch in _chunks(video_ids, 50):
+            data = self._get(
+                "videos",
+                {
+                    "part": "snippet,contentDetails,statistics,topicDetails,liveStreamingDetails",
+                    "id": ",".join(batch),
+                    "maxResults": 50,
+                },
+            )
+            items.extend(data.get("items", []))
+        return items