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

backend/src/utils/read/avatars.py

@@ -21,7 +21,7 @@ WHERE account_id = $1 AND owner_id = $2 AND unique_id = $3
 _AVATAR_HISTORY = """
 SELECT unique_id, first_seen_at, downloaded FROM avatars
 WHERE account_id = $1 AND owner_id = $2
-ORDER BY first_seen_at DESC
+ORDER BY first_seen_at DESC LIMIT 200
 """