feat(server): 单域托管 client/dist 与 SPA 回退（E12）

- 新增 SERVE_CLIENT_DIST/CLIENT_DIST_DIR 配置
- 开启后注册 @fastify/static 并对非 /api GET(html) 回退 index.html
- docker-compose.yml 与 README 补充用法

docker-compose.yml

@@ -25,6 +25,8 @@ services:
       JWT_SECRET: "change_me"
       APP_TIMEZONE: "Asia/Shanghai"
       STORAGE_DIR: "generated/storage"
+      SERVE_CLIENT_DIST: "false"
+      CLIENT_DIST_DIR: "../client/dist"
       SIGNED_URL_TTL_SECONDS: "600"
       MAX_UPLOAD_SIZE_BYTES: "10485760"
       PORT: "3000"