feat(ai): 完善AI网关服务并添加LLM降级模式

config.py 加openai_api_key/base_url/dev_mode

新建llm_client.py httpx异步调OpenAI REST API

main.py 业务路由加/ai前缀+降级模式+readyz端点

Gateway添加/notifications和/ai路由

docs: known-issues记录P5三服务经验

services/api-gateway/internal/config/config.go

@@ -16,6 +16,8 @@ type Config struct {
 	CoreEduServiceURL string
 	ContentServiceURL string
 	DataAnaServiceURL string
+	MsgServiceURL     string
+	AiServiceURL      string
 	OTLPEndpoint      string
 	LogLevel          string
 	DevMode           bool
@@ -33,6 +35,8 @@ func Load() *Config {
 		CoreEduServiceURL: getEnv("CORE_EDU_SERVICE_URL", "http://localhost:3004"),
 		ContentServiceURL: getEnv("CONTENT_SERVICE_URL", "http://localhost:3005"),
 		DataAnaServiceURL: getEnv("DATA_ANA_SERVICE_URL", "http://localhost:3006"),
+		MsgServiceURL:     getEnv("MSG_SERVICE_URL", "http://localhost:3007"),
+		AiServiceURL:      getEnv("AI_SERVICE_URL", "http://localhost:3008"),
 		OTLPEndpoint:      getEnv("OTEL_EXPORTER_OTLP_ENDPOINT", "http://localhost:4318"),
 		LogLevel:          getEnv("LOG_LEVEL", "info"),
 		DevMode:           getEnvBool("DEV_MODE", false),