feat: add cache-user-id toggle for Claude cloaking

Default to generating a fresh random user_id per request instead of
reusing cached IDs. Add cache-user-id config option to opt in to the
previous caching behavior.

- Add CacheUserID field to CloakConfig
- Extract user_id cache logic to dedicated file
- Generate fresh user_id by default, cache only when enabled
- Add tests for both paths

internal/config/config.go

@@ -301,6 +301,10 @@ type CloakConfig struct {
 	// SensitiveWords is a list of words to obfuscate with zero-width characters.
 	// This can help bypass certain content filters.
 	SensitiveWords []string `yaml:"sensitive-words,omitempty" json:"sensitive-words,omitempty"`
+
+	// CacheUserID controls whether Claude user_id values are cached per API key.
+	// When false, a fresh random user_id is generated for every request.
+	CacheUserID *bool `yaml:"cache-user-id,omitempty" json:"cache-user-id,omitempty"`
 }
 
 // ClaudeKey represents the configuration for a Claude API key,