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/config.py

@@ -131,6 +131,31 @@ class Settings(BaseSettings):
     # ("was_live") are real watchable content and stay visible.
     feed_default_hidden_live: str = "live,upcoming"
 
+    # --- Download center (yt-dlp) ---
+    # Root directory (inside the container) for downloaded media — a Docker volume/bind mount.
+    # Env-only (a mount path can't live in the DB). The worker lays a Plex-style tree under it.
+    download_root: str = "/downloads"
+    # Whether THIS process runs the download worker loop. On for the dedicated worker
+    # container, off for the API (which keeps the scheduler). Mirrors scheduler_enabled.
+    worker_enabled: bool = False
+    # How many downloads the worker runs concurrently (claimed via FOR UPDATE SKIP LOCKED).
+    download_worker_concurrency: int = 2
+    # How often (empty pending queue) the worker polls for a new job, in seconds.
+    download_poll_seconds: int = 5
+    # Default per-user quota (admin-tunable; a per-user download_quotas row overrides).
+    download_default_max_bytes: int = 5_368_709_120  # 5 GiB
+    download_default_max_concurrent: int = 2
+    download_default_max_jobs: int = 50
+    # Retention: an asset expires this many days after its last access; the GC job then
+    # deletes it (once no job references it) after an additional grace window.
+    download_retention_days: int = 30
+    download_gc_grace_days: int = 2
+    download_gc_minutes: int = 360
+    # On-disk layout: "plex" (Channel/Season YYYY/… [id].ext + .nfo/poster) or "flat".
+    download_layout: str = "plex"
+    # Default SponsorBlock (skip sponsor segments) for new custom profiles.
+    sponsorblock_default: bool = False
+
     @property
     def allowed_email_set(self) -> set[str]:
         return {e.strip().lower() for e in self.allowed_emails.split(",") if e.strip()}