- C3: `_reference_url` (downloads.py) and public.py's inline source-URL block were
the same rule → extract `service.reference_url(job, asset)`; both surfaces now
share it so the "downloaded from" link can't drift between them.
- C4: Content-Disposition filename derivation (ext pick + doubled-ext strip + join)
was duplicated in download_file and watch_file → extract
`storage.download_filename(display_name, container, path)`.
- C5: inline the one-line `_clean_basename` passthrough (folded into C4).
- C6: the `db.get(MediaAsset, job.asset_id) if job.asset_id else None; _serialize(...)`
resolve-then-serialize dance was repeated across 6 single-job handlers → fold into
`_serialize_job(db, job)`. (File-serving handlers that use the asset for their own
checks keep their explicit resolve.)
Behavior-neutral; ruff/parse clean, localdev boots, downloads routes load.
cac5526399