chore(downloads): C3-C6 — DRY the source-URL, filename, and serialize helpers

- 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.

backend/app/routes/public.py

@@ -118,11 +118,7 @@ def watch_file(token: str, g: str | None = None, db: Session = Depends(get_db)):
     if path is None:
         raise HTTPException(status_code=404, detail="This video is no longer available.")
 
-    ext = asset.container or path.suffix.lstrip(".")
-    base = storage.display_filename(asset.title or "video")
-    if base.lower().endswith(f".{ext.lower()}"):
-        base = base[: -(len(ext) + 1)]
-    filename = f"{base}.{ext}"
+    filename = storage.download_filename(asset.title or "video", asset.container, path)
     disposition = "attachment" if link.allow_download else "inline"
     # Starlette's FileResponse honours the Range header (206) for seeking/scrubbing.
     return FileResponse(path, filename=filename, content_disposition_type=disposition)