perf(*): bound list queries, batch fetches, harden media downloads

backend/src/userbot/modules/capture/repository.py

@@ -122,7 +122,7 @@ async def max_message_id(
     pool: asyncpg.Pool, account_id: int, chat_id: int
 ) -> int | None:
     return await pool.fetchval(
-        "SELECT max(message_id) FROM messages WHERE account_id = $1 AND chat_id = $2",
+        "SELECT last_message_id FROM chat_stats WHERE account_id = $1 AND chat_id = $2",
         account_id,
         chat_id,
     )