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/models/__init__.py

@@ -2,7 +2,16 @@ from app.models.app_settings import AppSetting
 from app.models.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
+from app.models.download_job import DownloadJob
 from app.models.oauth_credentials import OAuthCredentials
 from app.models.video import Video
 
-__all__ = ["AppSetting", "Category", "Channel", "channel_categories", "OAuthCredentials", "Video"]
+__all__ = [
+    "AppSetting",
+    "Category",
+    "Channel",
+    "channel_categories",
+    "DownloadJob",
+    "OAuthCredentials",
+    "Video",
+]