Implement Phase 6: MeTube download integration

- MeTubeClient encapsulates all MeTube HTTP/Socket.IO calls (verified against
  the real MeTube source: /add returns no job id, GET /history gives a queue
  snapshot for reconciliation, filenames arrive already relative, percent is
  a 0-100 float)
- download_jobs table + service: request/dedup active downloads, apply live
  Socket.IO events (added/updated/completed/canceled/cleared) matched by
  canonical YouTube URL, safe relative-path -> public media URL construction
- Reconciliation on startup against MeTube's live queue/done state (section 19):
  non-terminal jobs recovered where possible, else marked "unknown"; already
  completed jobs are left untouched
- POST/GET /api/videos/{id}/download(-status), recheck-local; feed/video
  detail now report real local availability instead of a stub
- Frontend: download button with live status polling (queued/downloading %/
  postprocessing/completed/failed+retry), local <video> playback with
  YouTube fallback on playback error
- health.py now delegates to MeTubeClient (single place for MeTube calls)

26 new backend tests (63 total). Verified live: Socket.IO connects
successfully to the real MeTube instance on deploy.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/api/feed.py

@@ -10,6 +10,7 @@ from app.db import get_db
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
 from app.models.video import Video
+from app.services.download_jobs import latest_jobs_map
 from app.services.video_presentation import channel_categories_map, serialize_video
 
 router = APIRouter(dependencies=[Depends(require_session)])
@@ -76,12 +77,15 @@ def get_feed(
     channel_ids = list({v.channel_id for v in rows})
     channels = {c.id: c for c in db.query(Channel).filter(Channel.id.in_(channel_ids)).all()}
     categories_map = channel_categories_map(db, channel_ids)
+    jobs_map = latest_jobs_map(db, [v.id for v in rows])
 
     items = []
     for video in rows:
         channel = channels.get(video.channel_id)
         if channel is None:
             continue
-        items.append(serialize_video(video, channel, categories_map.get(channel.id, [])))
+        items.append(
+            serialize_video(video, channel, categories_map.get(channel.id, []), jobs_map.get(video.id))
+        )
 
     return {"items": items, "next_cursor": next_cursor}