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

backend/src/utils/jobs.py

@@ -9,12 +9,22 @@ JOBS_CHANGED_CHANNEL = "jobs_changed"
 async def enqueue(
     pool: asyncpg.Pool, account_id: int, kind: str, params: dict[str, Any]
 ) -> int:
+    params_json = json.dumps(params)
+    existing = await pool.fetchval(
+        "SELECT id FROM jobs WHERE account_id = $1 AND kind = $2 "
+        "AND params = $3::jsonb AND status IN ('pending', 'running') LIMIT 1",
+        account_id,
+        kind,
+        params_json,
+    )
+    if existing is not None:
+        return existing
     job_id = await pool.fetchval(
         "INSERT INTO jobs (account_id, kind, params) "
         "VALUES ($1, $2, $3::jsonb) RETURNING id",
         account_id,
         kind,
-        json.dumps(params),
+        params_json,
     )
     await pool.execute(f"NOTIFY {JOBS_CHANGED_CHANNEL}")
     return job_id