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/services/video_presentation.py

@@ -4,6 +4,7 @@ from sqlalchemy.orm import Session
 from app.models.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
+from app.models.download_job import DownloadJob
 from app.models.video import Video
 
 
@@ -21,7 +22,22 @@ def channel_categories_map(db: Session, channel_ids: list[int]) -> dict[int, lis
     return result
 
 
-def serialize_video(video: Video, channel: Channel, categories: list[dict]) -> dict:
+def _serialize_local(job: DownloadJob | None) -> dict:
+    if job is None:
+        return {"available": False, "status": "not_downloaded", "progress_percent": None, "media_url": None}
+    if job.status == "completed":
+        return {"available": True, "status": "completed", "progress_percent": 100, "media_url": job.media_url}
+    return {
+        "available": False,
+        "status": job.status,
+        "progress_percent": job.progress_percent,
+        "media_url": None,
+    }
+
+
+def serialize_video(
+    video: Video, channel: Channel, categories: list[dict], download_job: DownloadJob | None = None
+) -> dict:
     return {
         "youtube_video_id": video.youtube_video_id,
         "title": video.title,
@@ -37,10 +53,5 @@ def serialize_video(video: Video, channel: Channel, categories: list[dict]) -> d
         "duration_seconds": video.duration_seconds,
         "youtube_url": video.youtube_url,
         "categories": categories,
-        "local": {
-            "available": False,
-            "status": "not_downloaded",
-            "progress_percent": None,
-            "media_url": None,
-        },
+        "local": _serialize_local(download_job),
     }