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/components/VideoCard.tsx

@@ -1,6 +1,7 @@
 import { Link } from 'react-router-dom'
 import type { FeedVideoDto } from '../api/client'
 import { formatDuration, formatRelativeTime } from '../utils/format'
+import DownloadButton from './DownloadButton'
 
 interface Props {
   video: FeedVideoDto
@@ -31,6 +32,7 @@ function VideoCard({ video }: Props) {
           <Link to={href} className="button-link">
             Смотреть
           </Link>
+          <DownloadButton video={video} />
         </div>
       </div>
     </li>