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/DownloadButton.tsx

@@ -0,0 +1,87 @@
+import { useEffect, useState } from 'react'
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
+import type { FeedVideoDto } from '../api/client'
+import { downloadVideo, getDownloadStatus } from '../api/client'
+
+const ACTIVE_STATUSES = ['queued', 'downloading', 'postprocessing']
+
+const STATUS_LABELS: Record<string, string> = {
+  queued: 'В очереди...',
+  postprocessing: 'Обработка...',
+}
+
+interface Props {
+  video: FeedVideoDto
+}
+
+function DownloadButton({ video }: Props) {
+  const queryClient = useQueryClient()
+  const [polling, setPolling] = useState(ACTIVE_STATUSES.includes(video.local.status))
+
+  const statusQuery = useQuery({
+    queryKey: ['download-status', video.youtube_video_id],
+    queryFn: () => getDownloadStatus(video.youtube_video_id),
+    enabled: polling,
+    refetchInterval: (query) => (query.state.data && ACTIVE_STATUSES.includes(query.state.data.status) ? 2000 : false),
+    initialData: polling
+      ? {
+          status: video.local.status,
+          progress_percent: video.local.progress_percent,
+          media_url: video.local.media_url,
+          error_message: null,
+        }
+      : undefined,
+  })
+
+  const downloadMutation = useMutation({
+    mutationFn: () => downloadVideo(video.youtube_video_id),
+    onSuccess: (data) => {
+      queryClient.setQueryData(['download-status', video.youtube_video_id], data)
+      setPolling(true)
+    },
+  })
+
+  const current = polling ? statusQuery.data : undefined
+  const status = current?.status ?? video.local.status
+  const percent = current?.progress_percent ?? video.local.progress_percent
+
+  useEffect(() => {
+    if (current && !ACTIVE_STATUSES.includes(current.status)) {
+      setPolling(false)
+      if (current.status === 'completed' || current.status === 'failed') {
+        queryClient.invalidateQueries({ queryKey: ['feed'] })
+      }
+    }
+  }, [current?.status, queryClient])
+
+  if (status === 'completed') {
+    return <span className="download-badge completed">✓ На сервере</span>
+  }
+
+  if (status === 'downloading') {
+    return <span className="download-badge">{percent ?? 0}%</span>
+  }
+
+  if (status === 'queued' || status === 'postprocessing') {
+    return <span className="download-badge">{STATUS_LABELS[status]}</span>
+  }
+
+  if (status === 'failed') {
+    return (
+      <span className="download-error">
+        <span className="download-badge error">Ошибка</span>
+        <button onClick={() => downloadMutation.mutate()} disabled={downloadMutation.isPending}>
+          Повторить
+        </button>
+      </span>
+    )
+  }
+
+  return (
+    <button onClick={() => downloadMutation.mutate()} disabled={downloadMutation.isPending}>
+      ⬇ Скачать
+    </button>
+  )
+}
+
+export default DownloadButton