feat(downloads): M3 — per-user quota + retention GC

- downloads/quota.py: per-user limits (DownloadQuota row or sysconfig defaults; unlimited
  bypass), footprint = distinct ready-asset bytes the user holds, check_enqueue (max_jobs +
  max_bytes hard at enqueue), at_concurrency_limit (worker-side max_concurrent), usage snapshot
- downloads/gc.py: run_download_gc — pre-expiry warning (once, gc_notified flag), TTL deletion
  (files + row; FK SET NULL orphans holders' jobs -> 'expired'), LRU eviction over a total-cache
  cap; notifies holders on each event
- migration 0038: media_assets.gc_notified
- config/sysconfig: download_total_max_bytes (0=unlimited) in the downloads group
- service.enqueue enforces quota; worker claim skips users at their concurrency limit
- scheduler: download_gc job registered (default 360 min, admin-tunable)

Verified on localdev: footprint accounting, max_jobs block, pre-expiry warning (notifies all
holders), TTL deletion (asset+files gone, dirs pruned, jobs orphaned, holders notified).

backend/app/models.py

@@ -731,6 +731,10 @@ class MediaAsset(Base, TimestampMixin):
     nfo_written: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false"
     )
+    # Set once the GC has warned holders of the upcoming expiry, so it doesn't warn every run.
+    gc_notified: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false"
+    )
     # Denormalized naming/metadata for the Plex tree + ad-hoc display.
     title: Mapped[str | None] = mapped_column(Text)
     uploader: Mapped[str | None] = mapped_column(String(255))