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>

frontend/src/App.css

@@ -359,13 +359,49 @@
   margin-bottom: 16px;
 }
 
-.player-wrapper iframe {
+.player-wrapper iframe,
+.player-wrapper video {
   position: absolute;
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
   border: 0;
+  background: #000;
+}
+
+.download-badge {
+  display: inline-block;
+  font-size: 12px;
+  padding: 3px 8px;
+  border-radius: 10px;
+  background: #eee;
+}
+
+.download-badge.completed {
+  background: #dcf5df;
+  color: #1a7f2e;
+}
+
+.download-badge.error {
+  background: #fbdcdc;
+  color: #a3221f;
+}
+
+.download-error {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+}
+
+.video-actions button,
+.video-page-meta button {
+  font-size: 13px;
+  padding: 4px 10px;
+  border: 1px solid #ccc;
+  border-radius: 6px;
+  background: transparent;
+  cursor: pointer;
 }
 
 .video-page-title {