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

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

@@ -48,6 +48,16 @@ async def requeue_running(pool: asyncpg.Pool, account_id: int) -> int:
     return int(result.split()[-1])
 
 
+async def prune_finished(pool: asyncpg.Pool, account_id: int) -> int:
+    result = await pool.execute(
+        "DELETE FROM jobs WHERE account_id = $1 "
+        "AND status IN ('done', 'canceled', 'failed') "
+        "AND finished_at < now() - interval '30 days'",
+        account_id,
+    )
+    return int(result.split()[-1])
+
+
 async def save_cursor(pool: asyncpg.Pool, job_id: int, cursor: dict[str, Any]) -> None:
     await pool.execute(
         "UPDATE jobs SET cursor = $2::jsonb, updated_at = now() WHERE id = $1",