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

src/main/resources/application.properties

@@ -1,2 +1,10 @@
 server.port=8082
 spring.application.name=springboot-demo
+
+# 学习友好：默认只保护 /api/secure/**
+learning.auth.enabled=true
+learning.auth.jwt.secret=demo-learning-secret-key-demo-learning-secret-key
+learning.auth.jwt.expiration=86400000
+
+# 避免默认生成密码干扰学习输出
+spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration