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/pages/VideoPage.tsx

@@ -2,6 +2,8 @@ import { useQuery } from '@tanstack/react-query'
 import { Link, useParams } from 'react-router-dom'
 import { getVideo } from '../api/client'
 import { formatDuration, formatRelativeTime } from '../utils/format'
+import DownloadButton from '../components/DownloadButton'
+import Player from '../components/Player'
 
 function VideoPage() {
   const { youtubeVideoId } = useParams<{ youtubeVideoId: string }>()
@@ -40,14 +42,7 @@ function VideoPage() {
         ← Лента
       </Link>
 
-      <div className="player-wrapper">
-        <iframe
-          src={`https://www.youtube-nocookie.com/embed/${video.youtube_video_id}`}
-          title={video.title}
-          allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
-          allowFullScreen
-        />
-      </div>
+      <Player video={video} />
 
       <h1 className="video-page-title">{video.title}</h1>
 
@@ -56,10 +51,10 @@ function VideoPage() {
         {' · '}
         {formatRelativeTime(video.published_at)}
         {duration && ` · ${duration}`}
+        {' · '}
+        <DownloadButton video={video} />
       </div>
 
-      <div className="video-page-source">Источник: YouTube</div>
-
       {video.categories.length > 0 && (
         <div className="video-page-categories">
           {video.categories.map((c) => (