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>

backend/app/api/health.py

@@ -1,12 +1,11 @@
 import logging
 
-import httpx
 from fastapi import APIRouter, Depends
 from sqlalchemy import text
 from sqlalchemy.orm import Session
 
-from app.config import settings
 from app.db import get_db
+from app.services.metube_client import MeTubeClient
 
 logger = logging.getLogger(__name__)
 
@@ -22,21 +21,10 @@ def _check_database(db: Session) -> str:
         return "error"
 
 
-def _check_metube() -> str:
-    try:
-        response = httpx.get(settings.metube_api_base_url, timeout=5)
-        if response.status_code < 500:
-            return "ok"
-        return "error"
-    except Exception:
-        logger.warning("MeTube healthcheck failed", exc_info=True)
-        return "error"
-
-
 @router.get("/health")
 def health(db: Session = Depends(get_db)) -> dict:
     return {
         "status": "ok",
         "database": _check_database(db),
-        "metube": _check_metube(),
+        "metube": "ok" if MeTubeClient().health() else "error",
     }