feat: add Docker deployment and Microsoft OAuth support

mengpost-backend/services/appcfg.go

@@ -0,0 +1,10 @@
+package services
+
+import "mengpost-backend/config"
+
+// AppCfg 供 IMAP/SMTP 刷新微软令牌等使用（main 启动时注入）.
+var AppCfg *config.Config
+
+func SetAppConfig(c *config.Config) {
+	AppCfg = c
+}