fix: 让消息 cache_control 改写默认关闭

backend/internal/handler/dto/settings.go

@@ -162,6 +162,7 @@ type SystemSettings struct {
 	EnableMetadataPassthrough          bool `json:"enable_metadata_passthrough"`
 	EnableCCHSigning                   bool `json:"enable_cch_signing"`
 	EnableAnthropicCacheTTL1hInjection bool `json:"enable_anthropic_cache_ttl_1h_injection"`
+	RewriteMessageCacheControl         bool `json:"rewrite_message_cache_control"`
 
 	// Web Search Emulation
 	WebSearchEmulationEnabled bool `json:"web_search_emulation_enabled"`