feat: add shared Keycloak compose baseline

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

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+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;
+    }
+}

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

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+package com.example.keycloakpattern;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class KeycloakPatternApplication {
+
+    public static void main(String[] args) {
+        SpringApplication.run(KeycloakPatternApplication.class, args);
+    }
+}

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

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+package com.example.keycloakpattern;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.Customizer;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.config.http.SessionCreationPolicy;
+import org.springframework.security.web.SecurityFilterChain;
+
+@Configuration
+public class SecurityConfig {
+
+    @Bean
+    SecurityFilterChain apiSecurity(HttpSecurity http) throws Exception {
+        return http
+            .csrf(csrf -> csrf.disable())
+            .sessionManagement(session ->
+                session.sessionCreationPolicy(SessionCreationPolicy.STATELESS))
+            .authorizeHttpRequests(authorize -> authorize
+                .requestMatchers("/actuator/health", "/actuator/health/**", "/api/public")
+                .permitAll()
+                .anyRequest()
+                .authenticated())
+            .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()))
+            .build();
+    }
+}

backend/src/main/resources/application.yml

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+server:
+  port: ${SERVER_PORT:8081}
+
+spring:
+  application:
+    name: keycloak-pattern-api
+  security:
+    oauth2:
+      resourceserver:
+        jwt:
+          issuer-uri: ${SPRING_SECURITY_OAUTH2_RESOURCESERVER_JWT_ISSUER_URI:http://localhost:8080/realms/keycloak-patterns}
+          jwk-set-uri: ${SPRING_SECURITY_OAUTH2_RESOURCESERVER_JWT_JWK_SET_URI:http://localhost:8080/realms/keycloak-patterns/protocol/openid-connect/certs}
+
+management:
+  endpoint:
+    health:
+      probes:
+        enabled: true
+  endpoints:
+    web:
+      exposure:
+        include: health,info

backend/src/test/java/com/example/keycloakpattern/ApiSecurityTest.java

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+package com.example.keycloakpattern;
+
+import static org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.jwt;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.web.servlet.MockMvc;
+
+@SpringBootTest
+@AutoConfigureMockMvc
+class ApiSecurityTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @Test
+    void publicEndpointDoesNotRequireAuthentication() throws Exception {
+        mockMvc.perform(get("/api/public"))
+            .andExpect(status().isOk())
+            .andExpect(jsonPath("$.status").value("ok"));
+    }
+
+    @Test
+    void protectedEndpointRejectsAnonymousRequests() throws Exception {
+        mockMvc.perform(get("/api/me"))
+            .andExpect(status().isUnauthorized());
+    }
+
+    @Test
+    void protectedEndpointAcceptsJwtAuthentication() throws Exception {
+        mockMvc.perform(get("/api/me").with(jwt().jwt(token -> token
+                .subject("test-subject")
+                .claim("preferred_username", "regular-user"))))
+            .andExpect(status().isOk())
+            .andExpect(jsonPath("$.subject").value("test-subject"))
+            .andExpect(jsonPath("$.username").value("regular-user"));
+    }
+}