feat(learning-auth): add optional JWT learning flow with secure demo endpoint

src/main/java/com/example/demo/dto/auth/LoginRequest.java

@@ -0,0 +1,8 @@
+package com.example.demo.dto.auth;
+
+import jakarta.validation.constraints.NotBlank;
+
+public record LoginRequest(
+        @NotBlank(message = "用户名不能为空") String username,
+        @NotBlank(message = "密码不能为空") String password
+) {}