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/worker.py

@@ -203,7 +203,8 @@ def _finish_from_cache(job_id: int, asset_id: int) -> None:
             job.progress = 100
             job.phase = None
             if not job.display_name:
-                job.display_name = asset.title
+                # display_name is VARCHAR(255); a title can be longer (e.g. a recipe in the title).
+                job.display_name = asset.title[:255] if asset.title else None
             asset.last_access_at = datetime.now(timezone.utc)
         db.commit()
 
@@ -425,7 +426,8 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                 job.speed_bps = None
                 job.eta_s = None
                 if not job.display_name:
-                    job.display_name = meta.title
+                    # display_name is VARCHAR(255) — clip a long title (see _fill_asset_meta_early).
+                    job.display_name = meta.title[:255] if meta.title else None
             db.commit()
         log.info("job %s done: %s", job_id, rel)