fix(playlists): the move's conflict fallback says why it is silent

The house guard requires a silent handler to carry a silent-ok note; the
savepoint fallback added in e74986cc did not, and the gate's pytest lane
caught it.

backend/app/routes/playlists.py

@@ -587,8 +587,10 @@ def move_item(
                 item.position = _next_item_position(db, dst.id)
                 db.flush()
             dst_changed = True
+        # silent-ok: the conflict IS the answer — the target already holds the video, which is the
+        # idempotent outcome; the savepoint put the row back in the source list, dropped below.
         except IntegrityError:
-            pass  # the savepoint put the row back in the source list; drop it below
+            pass
     if not dst_changed:
         db.delete(item)
     # The source always changed; the target only when the row landed in it. `_touch` on a target