security: add rate limiting to API server (V-016, CVSS 7.3)

Add token bucket rate limiter per client IP.

Changes:
- gateway/platforms/api_server.py:
  - Add _RateLimiter class with token bucket algorithm
  - Add rate_limit_middleware for request throttling
  - Configurable via API_SERVER_RATE_LIMIT (default 100 req/min)
  - Returns 429 with Retry-After header when limit exceeded
  - Skip rate limiting for /health endpoint

CVSS: 7.3 (High)
Refs: V-016 in SECURITY_AUDIT_REPORT.md
CWE-770: Allocation of Resources Without Limits or Throttling

gateway/platforms/api_server.py

@@ -241,6 +241,43 @@ else:
     security_headers_middleware = None  # type: ignore[assignment]
 
 
+# SECURITY FIX (V-016): Rate limiting middleware
+if AIOHTTP_AVAILABLE:
+    @web.middleware
+    async def rate_limit_middleware(request, handler):
+        """Apply rate limiting per client IP.
+        
+        Returns 429 Too Many Requests if rate limit exceeded.
+        Configurable via API_SERVER_RATE_LIMIT env var (requests per minute).
+        """
+        # Skip rate limiting for health checks
+        if request.path == "/health":
+            return await handler(request)
+        
+        # Get client IP (respecting X-Forwarded-For if behind proxy)
+        client_ip = request.headers.get("X-Forwarded-For", request.remote)
+        if client_ip and "," in client_ip:
+            client_ip = client_ip.split(",")[0].strip()
+        
+        limiter = _get_rate_limiter()
+        if not limiter.acquire(client_ip):
+            retry_after = limiter.get_retry_after(client_ip)
+            logger.warning(f"Rate limit exceeded for {client_ip}")
+            return web.json_response(
+                _openai_error(
+                    f"Rate limit exceeded. Try again in {retry_after} seconds.",
+                    err_type="rate_limit_error",
+                    code="rate_limit_exceeded"
+                ),
+                status=429,
+                headers={"Retry-After": str(retry_after)}
+            )
+        
+        return await handler(request)
+else:
+    rate_limit_middleware = None  # type: ignore[assignment]
+
+
 class _IdempotencyCache:
     """In-memory idempotency cache with TTL and basic LRU semantics."""
     def __init__(self, max_items: int = 1000, ttl_seconds: int = 300):
@@ -273,6 +310,59 @@ class _IdempotencyCache:
 _idem_cache = _IdempotencyCache()
 
 
+# SECURITY FIX (V-016): Rate limiting
+class _RateLimiter:
+    """Token bucket rate limiter per client IP.
+    
+    Default: 100 requests per minute per IP.
+    Configurable via API_SERVER_RATE_LIMIT env var (requests per minute).
+    """
+    def __init__(self, requests_per_minute: int = 100):
+        from collections import defaultdict
+        self._buckets = defaultdict(lambda: {"tokens": requests_per_minute, "last": 0})
+        self._rate = requests_per_minute / 60.0  # tokens per second
+        self._max_tokens = requests_per_minute
+        self._lock = threading.Lock()
+    
+    def _get_bucket(self, key: str) -> dict:
+        import time
+        with self._lock:
+            bucket = self._buckets[key]
+            now = time.time()
+            elapsed = now - bucket["last"]
+            bucket["last"] = now
+            # Add tokens based on elapsed time
+            bucket["tokens"] = min(
+                self._max_tokens,
+                bucket["tokens"] + elapsed * self._rate
+            )
+            return bucket
+    
+    def acquire(self, key: str) -> bool:
+        """Try to acquire a token. Returns True if allowed, False if rate limited."""
+        bucket = self._get_bucket(key)
+        with self._lock:
+            if bucket["tokens"] >= 1:
+                bucket["tokens"] -= 1
+                return True
+            return False
+    
+    def get_retry_after(self, key: str) -> int:
+        """Get seconds until next token is available."""
+        return 1  # Simplified - return 1 second
+
+
+_rate_limiter = None
+
+def _get_rate_limiter() -> _RateLimiter:
+    global _rate_limiter
+    if _rate_limiter is None:
+        # Parse rate limit from env (default 100 req/min)
+        rate_limit = int(os.getenv("API_SERVER_RATE_LIMIT", "100"))
+        _rate_limiter = _RateLimiter(rate_limit)
+    return _rate_limiter
+
+
 def _make_request_fingerprint(body: Dict[str, Any], keys: List[str]) -> str:
     from hashlib import sha256
     subset = {k: body.get(k) for k in keys}
@@ -1282,7 +1372,8 @@ class APIServerAdapter(BasePlatformAdapter):
             return False
 
         try:
-            mws = [mw for mw in (cors_middleware, body_limit_middleware, security_headers_middleware) if mw is not None]
+            # SECURITY FIX (V-016): Add rate limiting middleware
+            mws = [mw for mw in (cors_middleware, body_limit_middleware, security_headers_middleware, rate_limit_middleware) if mw is not None]
             self._app = web.Application(middlewares=mws)
             self._app["api_server_adapter"] = self
             self._app.router.add_get("/health", self._handle_health)