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>

migrations/versions/0005_download_jobs.py

@@ -0,0 +1,40 @@
+"""download_jobs
+
+Revision ID: 0005_download_jobs
+Revises: 0004_videos
+Create Date: 2026-09-16
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0005_download_jobs"
+down_revision: Union[str, None] = "0004_videos"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "download_jobs",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+        sa.Column("video_id", sa.Integer(), sa.ForeignKey("videos.id"), nullable=False),
+        sa.Column("status", sa.String(length=20), nullable=False, server_default="queued"),
+        sa.Column("metube_job_id", sa.String(length=255), nullable=True),
+        sa.Column("metube_filename", sa.String(), nullable=True),
+        sa.Column("media_url", sa.String(), nullable=True),
+        sa.Column("progress_percent", sa.Integer(), nullable=True),
+        sa.Column("error_message", sa.Text(), nullable=True),
+        sa.Column("requested_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("started_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+    )
+    op.create_index("ix_download_jobs_video_id", "download_jobs", ["video_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_download_jobs_video_id", table_name="download_jobs")
+    op.drop_table("download_jobs")