feat(downloads): M1 — schema, config, worker container, deps

- models: MediaAsset (shared file cache, unique on source+format_sig), DownloadJob
  (per-user request), DownloadProfile (presets), DownloadShare (ACL), DownloadQuota
- migrations 0036 (5 tables) + 0037 (6 builtin presets)
- sysconfig 'downloads' group (per-user defaults, retention/GC, layout, worker concurrency)
- config: DOWNLOAD_ROOT / WORKER_ENABLED env + download defaults
- deps: yt-dlp in requirements, ffmpeg in Dockerfile runtime stage
- worker: dedicated container (python -m app.worker), thin idle entry (loop lands in M2)
- localdev compose: worker service + downloads bind-mount; gitignore downloads/

backend/app/worker.py

@@ -0,0 +1,50 @@
+"""Download worker process entry point.
+
+Runs in a DEDICATED container (`command: python -m app.worker`, `WORKER_ENABLED=1`), separate
+from the API so long yt-dlp downloads + ffmpeg postprocessing never block web requests. It
+claims `queued` download jobs from the DB with FOR UPDATE SKIP LOCKED and runs them, writing
+live progress back to the job row (the frontend polls it — the worker is a separate process and
+can't reach the API's in-process WebSocket registry).
+
+M1: thin, idle entry point (keeps the container healthy). The claim loop + yt-dlp integration
+land in M2.
+"""
+import logging
+import signal
+import time
+
+from app.config import settings
+
+log = logging.getLogger("siftlode.worker")
+
+_stop = False
+
+
+def _handle_signal(signum, frame):
+    global _stop
+    _stop = True
+
+
+def main() -> None:
+    logging.basicConfig(level=logging.INFO)
+    signal.signal(signal.SIGTERM, _handle_signal)
+    signal.signal(signal.SIGINT, _handle_signal)
+
+    if not settings.worker_enabled:
+        log.info("WORKER_ENABLED is off; worker exiting (nothing to do).")
+        return
+
+    log.info(
+        "Download worker started (concurrency=%d, root=%s). Claim loop lands in M2.",
+        settings.download_worker_concurrency,
+        settings.download_root,
+    )
+    while not _stop:
+        # M2: claim queued jobs (FOR UPDATE SKIP LOCKED) and download them.
+        time.sleep(settings.download_poll_seconds)
+
+    log.info("Download worker stopped.")
+
+
+if __name__ == "__main__":
+    main()