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>

tests/test_health.py

@@ -21,8 +21,7 @@ def override_get_db():
 
 
 def test_health_ok():
-    with patch("app.api.health.httpx.get") as mock_get:
-        mock_get.return_value = MagicMock(status_code=200)
+    with patch("app.services.metube_client.MeTubeClient.health", return_value=True):
         client = TestClient(app)
         response = client.get("/api/health")
 
@@ -34,7 +33,7 @@ def test_health_ok():
 
 
 def test_health_metube_unreachable():
-    with patch("app.api.health.httpx.get", side_effect=ConnectionError):
+    with patch("app.services.metube_client.MeTubeClient.health", return_value=False):
         client = TestClient(app)
         response = client.get("/api/health")