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/alembic/versions/0041_download_edit.py

@@ -0,0 +1,66 @@
+"""download editor (phase 2): per-user trim/crop derivatives
+
+Revision ID: 0041_download_edit
+Revises: 0040_reset_profiles
+Create Date: 2026-07-04
+
+Adds the editor columns to `download_jobs`. An edit job (`job_kind='edit'`) derives a new per-user
+clip from an already-downloaded job by running ffmpeg (trim = stream-copy, crop = re-encode). It
+still hangs off a `media_assets` row (with `source_kind='edit'` and a per-user cache key) so the
+existing file-serve / ref-count / GC / quota machinery is reused unchanged. `source_job_id` /
+`source_asset_id` point at the parent download; `edit_spec` is the trim/crop recipe.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0041_download_edit"
+down_revision: Union[str, None] = "0040_reset_profiles"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "download_jobs",
+        sa.Column(
+            "job_kind", sa.String(length=16), server_default="download", nullable=False
+        ),
+    )
+    op.add_column(
+        "download_jobs", sa.Column("source_job_id", sa.Integer(), nullable=True)
+    )
+    op.add_column(
+        "download_jobs", sa.Column("source_asset_id", sa.Integer(), nullable=True)
+    )
+    op.add_column("download_jobs", sa.Column("edit_spec", sa.JSON(), nullable=True))
+    op.create_foreign_key(
+        "fk_download_jobs_source_job",
+        "download_jobs",
+        "download_jobs",
+        ["source_job_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+    op.create_foreign_key(
+        "fk_download_jobs_source_asset",
+        "download_jobs",
+        "media_assets",
+        ["source_asset_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "fk_download_jobs_source_asset", "download_jobs", type_="foreignkey"
+    )
+    op.drop_constraint(
+        "fk_download_jobs_source_job", "download_jobs", type_="foreignkey"
+    )
+    op.drop_column("download_jobs", "edit_spec")
+    op.drop_column("download_jobs", "source_asset_id")
+    op.drop_column("download_jobs", "source_job_id")
+    op.drop_column("download_jobs", "job_kind")