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/main.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 from contextlib import asynccontextmanager
 from pathlib import Path
@@ -29,18 +30,48 @@ from app.api.health import router as health_router
 from app.api.sync import router as sync_router
 from app.api.videos import router as videos_router
 from app.config import settings
+from app.db import SessionLocal
+from app.services import download_jobs
+from app.services.metube_client import MeTubeClient
 from app.services.scheduler import create_scheduler
 
 logging.basicConfig(level=settings.log_level)
 logger = logging.getLogger(__name__)
 
 
+async def _on_metube_event(event_name: str, payload) -> None:
+    db = SessionLocal()
+    try:
+        await download_jobs.handle_metube_event(db, event_name, payload)
+    except Exception:
+        logger.exception("Failed handling MeTube '%s' event", event_name)
+    finally:
+        db.close()
+
+
+def _reconcile_download_jobs() -> None:
+    db = SessionLocal()
+    try:
+        download_jobs.reconcile_on_startup(db)
+    except Exception:
+        logger.exception("Failed to reconcile download jobs on startup")
+    finally:
+        db.close()
+
+
 @asynccontextmanager
 async def lifespan(_: FastAPI):
     logger.info("Application startup")
+    _reconcile_download_jobs()
+
     scheduler = create_scheduler()
     scheduler.start()
+
+    metube_task = asyncio.create_task(MeTubeClient().run_event_listener(_on_metube_event))
+
     yield
+
+    metube_task.cancel()
     scheduler.shutdown(wait=False)
     logger.info("Application shutdown")