feat: add shared Keycloak compose baseline

backend/src/main/java/com/example/keycloakpattern/ApiController.java

@@ -0,0 +1,30 @@
+package com.example.keycloakpattern;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import org.springframework.security.core.annotation.AuthenticationPrincipal;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/api")
+public class ApiController {
+
+    @GetMapping("/public")
+    public Map<String, String> publicEndpoint() {
+        return Map.of("status", "ok", "service", "keycloak-pattern-api");
+    }
+
+    @GetMapping("/me")
+    public Map<String, Object> currentUser(@AuthenticationPrincipal Jwt jwt) {
+        Map<String, Object> response = new LinkedHashMap<>();
+        response.put("subject", jwt.getSubject());
+        response.put("username", jwt.getClaimAsString("preferred_username"));
+        response.put("issuer", jwt.getIssuer());
+        response.put("audience", jwt.getAudience());
+        return response;
+    }
+}