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/

.gitignore

@@ -21,6 +21,9 @@ frontend/dist/
 *.sql.gz
 backups/
 
+# Download center: locally downloaded media (the DOWNLOAD_ROOT bind mount)
+downloads/
+
 # OS / editor
 .DS_Store
 Thumbs.db