Add test infrastructure and common module

- Testcontainers 2.0.4 (PostgreSQL) for repository integration tests
- AbstractIntegrationTest base class with shared Postgres container
- application-test.yml for test profile
- Common module: ApiResponse/ApiError envelopes, GlobalExceptionHandler,
  ResourceNotFoundException, ConflictException, ValidationException,
  HouseholdContext record

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

backend/src/test/java/com/recipeapp/AbstractIntegrationTest.java

@@ -0,0 +1,28 @@
+package com.recipeapp;
+
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.DynamicPropertyRegistry;
+import org.springframework.test.context.DynamicPropertySource;
+import org.testcontainers.containers.PostgreSQLContainer;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+
+@SpringBootTest
+@Testcontainers
+@ActiveProfiles("test")
+public abstract class AbstractIntegrationTest {
+
+    @Container
+    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine")
+            .withDatabaseName("mealprep")
+            .withUsername("mealprep")
+            .withPassword("mealprep");
+
+    @DynamicPropertySource
+    static void configureProperties(DynamicPropertyRegistry registry) {
+        registry.add("spring.datasource.url", postgres::getJdbcUrl);
+        registry.add("spring.datasource.username", postgres::getUsername);
+        registry.add("spring.datasource.password", postgres::getPassword);
+    }
+}