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/Player.tsx

@@ -0,0 +1,60 @@
+import { useState } from 'react'
+import { useMutation, useQueryClient } from '@tanstack/react-query'
+import type { FeedVideoDto } from '../api/client'
+import { recheckLocal } from '../api/client'
+
+interface Props {
+  video: FeedVideoDto
+}
+
+function Player({ video }: Props) {
+  const [localFailed, setLocalFailed] = useState(false)
+  const queryClient = useQueryClient()
+
+  const recheckMutation = useMutation({
+    mutationFn: () => recheckLocal(video.youtube_video_id),
+    onSettled: () => {
+      queryClient.invalidateQueries({ queryKey: ['video', video.youtube_video_id] })
+      queryClient.invalidateQueries({ queryKey: ['download-status', video.youtube_video_id] })
+    },
+  })
+
+  const useLocal = video.local.available && !!video.local.media_url && !localFailed
+
+  if (useLocal) {
+    return (
+      <>
+        <div className="player-wrapper">
+          {/* eslint-disable-next-line jsx-a11y/media-has-caption */}
+          <video
+            controls
+            src={video.local.media_url!}
+            onError={() => {
+              setLocalFailed(true)
+              recheckMutation.mutate()
+            }}
+          />
+        </div>
+        <div className="video-page-source">● Локальная копия · mediaVM</div>
+      </>
+    )
+  }
+
+  return (
+    <>
+      <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>
+      <div className="video-page-source">
+        {localFailed ? 'Локальная копия недоступна. Переключено на YouTube.' : 'Источник: YouTube'}
+      </div>
+    </>
+  )
+}
+
+export default Player