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/models/download_job.py

@@ -0,0 +1,31 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Integer, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+# queued -> downloading -> postprocessing -> completed
+#                                          -> failed
+# unknown: state could not be reconciled after a restart
+ACTIVE_STATUSES = ("queued", "downloading", "postprocessing")
+TERMINAL_STATUSES = ("completed", "failed")
+
+
+class DownloadJob(Base):
+    __tablename__ = "download_jobs"
+
+    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+    video_id: Mapped[int] = mapped_column(ForeignKey("videos.id"), nullable=False, index=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="queued")
+    metube_job_id: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    metube_filename: Mapped[str | None] = mapped_column(String, nullable=True)
+    media_url: Mapped[str | None] = mapped_column(String, nullable=True)
+    progress_percent: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    error_message: Mapped[str | None] = mapped_column(Text, nullable=True)
+    requested_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    started_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    completed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False
+    )