fix(downloads): clamp notification title to column limit so download_gc can't wedge

A downloaded FB/IG reel's "title" is the whole caption (up to ~1900 chars). It flowed
untrimmed into Notification.title (String(255)); at the batched download_gc commit that
raised psycopg StringDataRightTruncation in pass 1, aborting the whole run so expiry
deletion, LRU eviction and orphan reap never executed (13/110 prod assets already exceed
the limit). Clamp the title to 255 (with an ellipsis) centrally in create_notification —
defending all six callers — since the full title already reaches the client via `data`
and the column is only an English fallback. Add a regression test.

backend/app/notifications.py

@@ -14,6 +14,13 @@ from app.models import Notification
 # rows are never pruned (the user hasn't seen them yet).
 READ_SOFT_CAP = 200
 
+# Hard limit of the Notification.title column (String(255)). Titles are only an English
+# fallback — the client renders the translated message from `data` — so clamping an
+# over-long one (e.g. a FB/IG reel whose "title" is the whole caption, hundreds of chars)
+# is harmless to the UI and prevents a StringDataRightTruncation that would abort the
+# caller's whole transaction (the batched download_gc run in particular).
+TITLE_MAX_LEN = 255
+
 
 def create_notification(
     db: Session,
@@ -27,6 +34,8 @@ def create_notification(
 ) -> Notification:
     """Insert one notification for a user. Callers that batch many inserts in a single
     transaction can pass commit=False and commit once themselves."""
+    if title is not None and len(title) > TITLE_MAX_LEN:
+        title = title[: TITLE_MAX_LEN - 1] + "…"  # clamp to the column limit, add an ellipsis
     notif = Notification(
         user_id=user_id, type=type, title=title, body=body, data=data
     )