fix: monthly reset now updates reset_at correctly

Critical bug: usage limits were never resetting after first month because
reset_at timestamp was not updated during ON CONFLICT UPDATE.

This caused users to stay permanently blocked after reaching monthly limit once.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/auth.py

@@ -318,8 +318,9 @@ def increment_feature_usage(profile_id: str, feature_id: str) -> None:
             ON CONFLICT (profile_id, feature_id)
             DO UPDATE SET
                 usage_count = user_feature_usage.usage_count + 1,
+                reset_at = %s,
                 updated = CURRENT_TIMESTAMP
-        """, (profile_id, feature_id, next_reset))
+        """, (profile_id, feature_id, next_reset, next_reset))
 
         conn.commit()