Fix requests cache migration on legacy databases

backend/app/db.py

@@ -418,12 +418,16 @@ def init_db() -> None:
             ON requests_cache (updated_at DESC, request_id DESC)
             """
         )
-        conn.execute(
-            """
-            CREATE INDEX IF NOT EXISTS idx_requests_cache_requested_by_id_created_at
-            ON requests_cache (requested_by_id, created_at DESC, request_id DESC)
-            """
-        )
+        try:
+            conn.execute(
+                """
+                CREATE INDEX IF NOT EXISTS idx_requests_cache_requested_by_id_created_at
+                ON requests_cache (requested_by_id, created_at DESC, request_id DESC)
+                """
+            )
+        except sqlite3.OperationalError:
+            # Older databases may not have requested_by_id until later migrations run.
+            pass
         conn.execute(
             """
             CREATE INDEX IF NOT EXISTS idx_requests_cache_requested_by_norm_created_at