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

src/main/java/com/example/demo/controller/auth/SecureDemoController.java

@@ -0,0 +1,23 @@
+package com.example.demo.controller.auth;
+
+import com.example.demo.common.ApiResponse;
+import org.springframework.security.core.Authentication;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.util.Map;
+
+@RestController
+@RequestMapping("/api/secure")
+public class SecureDemoController {
+
+    @GetMapping("/me")
+    public ApiResponse<Map<String, Object>> me(Authentication authentication) {
+        return ApiResponse.ok(Map.of(
+                "principal", authentication.getName(),
+                "authorities", authentication.getAuthorities(),
+                "message", "你已通过学习用 JWT 鉴权"
+        ));
+    }
+}