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/models.py

@@ -779,7 +779,13 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     freezes the spec at enqueue so later edits/deletion of the profile don't change this job.
     `display_name` is the user's own name for the file (display + the Content-Disposition on
     local download); the on-disk asset is never renamed. `asset_id` links to the shared file
-    (SET NULL if the asset is GC'd — the job then shows as expired)."""
+    (SET NULL if the asset is GC'd — the job then shows as expired).
+
+    Phase 2 (editor): `job_kind='edit'` jobs derive a new per-user clip from an already-downloaded
+    ready job. They still hang off a `MediaAsset` (with `source_kind='edit'`, a per-user cache key)
+    so all the file-serve/ref-count/GC/quota machinery is reused; the worker branches on the
+    asset's `source_kind` and runs ffmpeg instead of yt-dlp. `source_job_id`/`source_asset_id`
+    point at the parent download the clip was cut from; `edit_spec` is the (trim/crop) recipe."""
 
     __tablename__ = "download_jobs"
     __table_args__ = (Index("ix_download_jobs_claim", "status", "queue_pos"),)
@@ -793,6 +799,19 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     )
     source_kind: Mapped[str] = mapped_column(String(16))  # "youtube" | "url"
     source_ref: Mapped[str] = mapped_column(String(512))
+    # "download" (default) or "edit" (a per-user trim/crop derivative of another job).
+    job_kind: Mapped[str] = mapped_column(
+        String(16), default="download", server_default="download"
+    )
+    # For edit jobs: the parent download this clip is cut from (SET NULL if the parent is deleted).
+    source_job_id: Mapped[int | None] = mapped_column(
+        ForeignKey("download_jobs.id", ondelete="SET NULL")
+    )
+    source_asset_id: Mapped[int | None] = mapped_column(
+        ForeignKey("media_assets.id", ondelete="SET NULL")
+    )
+    # For edit jobs: the trim/crop recipe {trim?:{start_s,end_s}, crop?:{x,y,w,h}}.
+    edit_spec: Mapped[dict | None] = mapped_column(JSON)
     profile_id: Mapped[int | None] = mapped_column(
         ForeignKey("download_profiles.id", ondelete="SET NULL")
     )