fix: add logging to rate limiter middleware

Log rate limiting events that were previously silent:
- WARNING when a request is throttled (429) with principal, limit
  type, current counts, and retry_after
- ERROR when Redis backend is unavailable (503)
- ERROR when Redis Lua eval fails

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

Author: luis5tb

src/lightspeed_agent/ratelimit/middleware.py

@@ -1,5 +1,6 @@
 """Redis-backed rate limiting middleware with global limits."""
 
+import logging
 import math
 import time
 import uuid
@@ -14,6 +15,8 @@
 
 from lightspeed_agent.config import get_settings
 
+logger = logging.getLogger(__name__)
+
 
 class RedisRateLimiter:
     """Distributed Redis rate limiter using atomic Lua + ZSET sliding windows."""
@@ -140,6 +143,7 @@ async def is_allowed(
                 unique_member,
             )
         except RedisError as exc:
+            logger.error("Redis rate limiter check failed: %s", exc)
             raise RuntimeError("Redis rate limiter check failed") from exc
 
         allowed = bool(int(result[0]))
@@ -231,6 +235,10 @@ async def dispatch(
         try:
             allowed, status = await self._limiter.is_allowed(principal_keys=principals)
         except RuntimeError:
+            logger.error(
+                "Rate limiter backend unavailable, returning 503 (principals=%s)",
+                principals,
+            )
             return JSONResponse(
                 status_code=503,
                 content={
@@ -240,6 +248,15 @@ async def dispatch(
             )
 
         if not allowed:
+            logger.warning(
+                "Rate limit exceeded: principal=%s, limit=%s, "
+                "requests_minute=%s, requests_hour=%s, retry_after=%s",
+                status.get("limited_principal"),
+                status.get("exceeded"),
+                status.get("requests_this_minute"),
+                status.get("requests_this_hour"),
+                status.get("retry_after"),
+            )
             return self._rate_limit_response(status)
 
         # Process request