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/api/client.ts

@@ -168,6 +168,28 @@ export function getVideo(youtubeVideoId: string) {
   return request<FeedVideoDto>(`/api/videos/${youtubeVideoId}`)
 }
 
+export interface DownloadJobDto {
+  status: string
+  progress_percent: number | null
+  media_url: string | null
+  error_message: string | null
+  requested_at?: string
+  started_at?: string | null
+  completed_at?: string | null
+}
+
+export function downloadVideo(youtubeVideoId: string) {
+  return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/download`, { method: 'POST' })
+}
+
+export function getDownloadStatus(youtubeVideoId: string) {
+  return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/download-status`)
+}
+
+export function recheckLocal(youtubeVideoId: string) {
+  return request<DownloadJobDto>(`/api/videos/${youtubeVideoId}/recheck-local`, { method: 'POST' })
+}
+
 export function getFeed(
   params: { categoryId?: number; uncategorized?: boolean; channelId?: number; cursor?: string } = {},
 ) {