feat(downloads): editor backend — trim/crop derivatives (phase 2)

Per-user trim/crop clips derived from a finished download via ffmpeg, riding the same
MediaAsset/DownloadJob/worker/quota/GC machinery (asset source_kind='edit', per-user cache key):
- migration 0041: download_jobs job_kind/source_job_id/source_asset_id/edit_spec
- app/downloads/edit.py: spec normalize+sig, ffmpeg trim/crop cmd (fast stream-copy vs frame-
  accurate re-encode, per-edit 'accurate' toggle), progress+cooperative-cancel runner, filmstrip
- service.enqueue_edit; worker _process_edit branch; storage.edit_rel_path (.edits/{user} tree)
- routes: POST /edit, GET /{id}/storyboard(.jpg)

backend/app/downloads/storage.py

@@ -87,6 +87,15 @@ def rel_path(meta: MediaMeta, ext: str, layout: str = "plex") -> str:
     return f"{channel}/Season_{year}/{epname}"
 
 
+def edit_rel_path(user_id: int, asset_id: int, ext: str) -> str:
+    """On-disk path (relative to DOWNLOAD_ROOT) for a phase-2 editor derivative.
+
+    Kept in a per-user `.edits/` tree — device-downloadable via the file endpoint, but NOT part of
+    the Plex library (these are user clips, not canonical episodes; there are no .nfo/poster
+    sidecars). The name is system-decided and bound to the derivative asset id."""
+    return f".edits/{user_id}/clip_{asset_id}.{ext}"
+
+
 def abs_path(download_root: str, rel: str) -> Path:
     return Path(download_root) / rel