fix(downloads): clip long titles to fit display_name (VARCHAR 255)

A download whose source title exceeded 255 chars (e.g. a recipe baked into a
Facebook video's title) failed with a StringDataRightTruncation: the worker
auto-fills an empty display_name from the title, which overflows the 255-char
column. Clip the title to 255 at every point it flows into display_name (worker
completion paths + enqueue); the full title is untouched on the asset. Pre-existing
latent bug, surfaced by an unusually long title.

Release v0.31.1.

backend/app/downloads/service.py

@@ -114,7 +114,7 @@ def enqueue(
         source_ref=source_ref,
         profile_id=profile_id,
         profile_snapshot=spec,
-        display_name=display_name or (asset.title if asset.status == "ready" else None),
+        display_name=display_name or (asset.title[:255] if asset.status == "ready" and asset.title else None),
         status="queued",
         queue_pos=_next_queue_pos(db),
     )