fix(execution): heal a user budget key that somehow has no expiry

The no-op branch of the flush script dropped the user-key EXPIRE outright
rather than guarding it like every other site. No current path can create
the key without an expiry, but if one ever did, that branch was the one
place that would never give it one — and a user counter with no expiry is
the unbounded version of the bug this series fixes.

Guard it instead, so the branch heals such a key rather than skipping it,
and so all five scripts read the same way.

Author: waleedlatif1

apps/sim/lib/execution/event-buffer.ts

@@ -109,6 +109,9 @@ else
   if redis.call('EXISTS', KEYS[4]) == 1 then
     redis.call('EXPIRE', KEYS[4], budget_ttl_seconds)
   end
+  if #KEYS >= 5 and redis.call('EXISTS', KEYS[5]) == 1 and redis.call('TTL', KEYS[5]) < 0 then
+    redis.call('EXPIRE', KEYS[5], budget_ttl_seconds)
+  end
 end
 for i = 9, #ARGV, 2 do
   redis.call('ZADD', KEYS[1], ARGV[i], ARGV[i + 1])