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/tests/test_notifications.py

@@ -0,0 +1,55 @@
+"""create_notification title-clamp regression.
+
+A downloaded FB/IG reel's "title" is the whole caption (hundreds to ~1900 chars). Before the
+clamp that string went straight into Notification.title (String(255)) and, at the batched
+download_gc commit, raised psycopg StringDataRightTruncation — aborting the entire GC run so
+no expiry deletion / LRU eviction / orphan reap ever ran. The clamp trims the title to the
+column limit centrally, defending all callers regardless of DB backend."""
+from app.models import Notification
+from app.notifications import TITLE_MAX_LEN, create_notification
+
+
+class _CaptureDB:
+    """Captures the added row without a real DB, so the clamp is verified on every lane
+    (the truncation only reproduces on Postgres; the clamp logic is backend-agnostic)."""
+
+    def __init__(self):
+        self.added: list[Notification] = []
+
+    def add(self, obj):
+        self.added.append(obj)
+
+    def commit(self):
+        pass
+
+    def refresh(self, _obj):
+        pass
+
+
+def test_title_clamped_to_column_limit():
+    long_title = "x" * 2000  # e.g. a reel caption
+    notif = create_notification(
+        _CaptureDB(), user_id=1, type="download_expiring", title=long_title
+    )
+    assert len(notif.title) == TITLE_MAX_LEN
+    assert notif.title.endswith("…")
+    # The untruncated title still reaches the client via `data`, which is never clamped.
+    notif2 = create_notification(
+        _CaptureDB(),
+        user_id=1,
+        type="download_expiring",
+        title=long_title,
+        data={"kind": "download_expiring", "title": long_title},
+    )
+    assert notif2.data["title"] == long_title
+
+
+def test_short_title_is_untouched():
+    notif = create_notification(_CaptureDB(), user_id=1, type="t", title="short")
+    assert notif.title == "short"
+
+
+def test_title_at_exactly_the_limit_is_untouched():
+    exact = "y" * TITLE_MAX_LEN
+    notif = create_notification(_CaptureDB(), user_id=1, type="t", title=exact)
+    assert notif.title == exact