T-4: add JPA, Liquibase, Testcontainers, and deployment docs

Set up development infrastructure for TDD: JPA + Liquibase for
database migrations, Testcontainers for integration tests against
real PostgreSQL, profile-based configuration (prod/local), and
README deployment documentation with docker-compose example.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/test/java/de/fete/FeteApplicationTest.java

@@ -8,10 +8,12 @@ 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.context.annotation.Import;
 import org.springframework.test.web.servlet.MockMvc;
 
 @SpringBootTest
 @AutoConfigureMockMvc
+@Import(TestcontainersConfig.class)
 class FeteApplicationTest {
 
   @Autowired

backend/src/test/java/de/fete/TestFeteApplication.java

@@ -0,0 +1,14 @@
+package de.fete;
+
+import org.springframework.boot.SpringApplication;
+
+/** Test entry point — starts the app with Testcontainers PostgreSQL. */
+public class TestFeteApplication {
+
+  /** Starts the application with Testcontainers PostgreSQL. */
+  public static void main(String[] args) {
+    SpringApplication.from(FeteApplication::main)
+        .with(TestcontainersConfig.class)
+        .run(args);
+  }
+}

backend/src/test/java/de/fete/TestcontainersConfig.java

@@ -0,0 +1,17 @@
+package de.fete;
+
+import org.springframework.boot.test.context.TestConfiguration;
+import org.springframework.boot.testcontainers.service.connection.ServiceConnection;
+import org.springframework.context.annotation.Bean;
+import org.testcontainers.containers.PostgreSQLContainer;
+
+/** Provides a Testcontainers PostgreSQL instance for integration tests. */
+@TestConfiguration(proxyBeanMethods = false)
+public class TestcontainersConfig {
+
+  @Bean
+  @ServiceConnection
+  PostgreSQLContainer<?> postgresContainer() {
+    return new PostgreSQLContainer<>("postgres:17-alpine");
+  }
+}

backend/src/test/java/de/fete/config/WebConfigTest.java

@@ -4,14 +4,17 @@ import static org.springframework.test.web.servlet.request.MockMvcRequestBuilder
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
+import de.fete.TestcontainersConfig;
 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.context.annotation.Import;
 import org.springframework.test.web.servlet.MockMvc;
 
 @SpringBootTest
 @AutoConfigureMockMvc
+@Import(TestcontainersConfig.class)
 class WebConfigTest {
 
   @Autowired