Fix memory leak in rate limiter

Empty bucket keys were never removed after their timestamps expired.
Over time this accumulated an entry for every unique client IP that
ever made a request.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

responder/ext/ratelimit.py

@@ -39,6 +39,8 @@ class RateLimiter:
         now = time.time()
         cutoff = now - self.period
         self._buckets[key] = [t for t in self._buckets[key] if t > cutoff]
+        if not self._buckets[key]:
+            del self._buckets[key]
 
     def check(self, req, resp):
         """Check rate limit. Sets 429 status if exceeded."""