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/rss.py

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+"""Free per-channel RSS feed reader (no API quota). Returns the channel's most recent
+~15 uploads as lightweight stubs for fast fresh-video detection."""
+from datetime import datetime, timezone
+
+import feedparser
+import httpx
+
+RSS_URL = "https://www.youtube.com/feeds/videos.xml?channel_id={channel_id}"
+
+
+def fetch_channel_feed(channel_id: str) -> list[dict]:
+    url = RSS_URL.format(channel_id=channel_id)
+    try:
+        resp = httpx.get(url, timeout=20.0, headers={"User-Agent": "Subfeed/1.0"})
+    except httpx.HTTPError:
+        return []
+    if resp.status_code != 200:
+        return []
+
+    parsed = feedparser.parse(resp.content)
+    out: list[dict] = []
+    for entry in parsed.entries:
+        video_id = entry.get("yt_videoid")
+        if not video_id:
+            continue
+        published = None
+        if entry.get("published_parsed"):
+            published = datetime(*entry.published_parsed[:6], tzinfo=timezone.utc)
+        thumbnail = None
+        media = entry.get("media_thumbnail") or []
+        if media:
+            thumbnail = media[0].get("url")
+        out.append(
+            {
+                "id": video_id,
+                "channel_id": channel_id,
+                "title": entry.get("title"),
+                "published_at": published,
+                "thumbnail_url": thumbnail,
+            }
+        )
+    return out