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/downloads/storage.py

@@ -96,6 +96,17 @@ def edit_rel_path(user_id: int, asset_id: int, ext: str) -> str:
     return f".edits/{user_id}/clip_{asset_id}.{ext}"
 
 
+def download_filename(display_name: str, container: str | None, path: Path) -> str:
+    """The Content-Disposition filename for a served download: the cleaned display name carrying a single
+    correct extension — the asset container, else the file's own suffix — without doubling it when the
+    name already ends in that extension. Shared by the private download and the public watch endpoints."""
+    ext = container or path.suffix.lstrip(".")
+    base = display_filename(display_name)
+    if base.lower().endswith(f".{ext.lower()}"):
+        base = base[: -(len(ext) + 1)]
+    return f"{base}.{ext}"
+
+
 def abs_path(download_root: str, rel: str) -> Path:
     return Path(download_root) / rel