fix: RateLimit graceful degradation on permission denied

Silence mkdir() with @ operator; guard file_put_contents with
is_writable() check. When storage/cache/rate_limit is not writable
by php-fpm, requests are allowed through instead of throwing
warnings that flood the nginx error log.

src/RateLimit.php

@@ -22,7 +22,7 @@ class RateLimit {
 
         // Create cache directory if it doesn't exist
         if (!is_dir($this->cacheDir)) {
-            mkdir($this->cacheDir, 0755, true);
+            @mkdir($this->cacheDir, 0755, true);
         }
     }
 
@@ -79,8 +79,10 @@ class RateLimit {
         // Add new request timestamp
         $data[] = $now;
 
-        // Save updated data
-        file_put_contents($file, json_encode($data));
+        // Save updated data (silently skip if directory is not writable)
+        if (is_dir($this->cacheDir) && is_writable($this->cacheDir)) {
+            file_put_contents($file, json_encode($data));
+        }
 
         return true;
     }