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/main/java/com/recipeapp/common/ApiError.java

@@ -0,0 +1,20 @@
+package com.recipeapp.common;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import java.util.List;
+
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public record ApiError(
+        String status,
+        ErrorBody error
+) {
+    public static ApiError of(String code, String message) {
+        return new ApiError("error", new ErrorBody(code, message, null));
+    }
+
+    public static ApiError of(String code, String message, List<String> details) {
+        return new ApiError("error", new ErrorBody(code, message, details));
+    }
+
+    public record ErrorBody(String code, String message, List<String> details) {}
+}

backend/src/main/java/com/recipeapp/common/ApiResponse.java

@@ -0,0 +1,22 @@
+package com.recipeapp.common;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public record ApiResponse<T>(
+        String status,
+        T data,
+        Meta meta
+) {
+    public static <T> ApiResponse<T> success(T data) {
+        return new ApiResponse<>("success", data, null);
+    }
+
+    public static <T> ApiResponse<T> success(T data, Meta meta) {
+        return new ApiResponse<>("success", data, meta);
+    }
+
+    public record Meta(Pagination pagination) {}
+
+    public record Pagination(long total, int limit, int offset, boolean hasMore) {}
+}

backend/src/main/java/com/recipeapp/common/ConflictException.java

@@ -0,0 +1,7 @@
+package com.recipeapp.common;
+
+public class ConflictException extends RuntimeException {
+    public ConflictException(String message) {
+        super(message);
+    }
+}

backend/src/main/java/com/recipeapp/common/GlobalExceptionHandler.java

@@ -0,0 +1,40 @@
+package com.recipeapp.common;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import java.util.List;
+
+@RestControllerAdvice
+public class GlobalExceptionHandler {
+
+    @ExceptionHandler(MethodArgumentNotValidException.class)
+    public ResponseEntity<ApiError> handleValidation(MethodArgumentNotValidException ex) {
+        List<String> details = ex.getBindingResult().getFieldErrors().stream()
+                .map(e -> e.getField() + ": " + e.getDefaultMessage())
+                .toList();
+        return ResponseEntity.status(HttpStatus.BAD_REQUEST)
+                .body(ApiError.of("VALIDATION_ERROR", "Validation failed", details));
+    }
+
+    @ExceptionHandler(ResourceNotFoundException.class)
+    public ResponseEntity<ApiError> handleNotFound(ResourceNotFoundException ex) {
+        return ResponseEntity.status(HttpStatus.NOT_FOUND)
+                .body(ApiError.of("NOT_FOUND", ex.getMessage()));
+    }
+
+    @ExceptionHandler(ConflictException.class)
+    public ResponseEntity<ApiError> handleConflict(ConflictException ex) {
+        return ResponseEntity.status(HttpStatus.CONFLICT)
+                .body(ApiError.of("CONFLICT", ex.getMessage()));
+    }
+
+    @ExceptionHandler(ValidationException.class)
+    public ResponseEntity<ApiError> handleBusinessValidation(ValidationException ex) {
+        return ResponseEntity.status(HttpStatus.UNPROCESSABLE_ENTITY)
+                .body(ApiError.of("VALIDATION_ERROR", ex.getMessage()));
+    }
+}

backend/src/main/java/com/recipeapp/common/HouseholdContext.java

@@ -0,0 +1,10 @@
+package com.recipeapp.common;
+
+import java.util.UUID;
+
+public record HouseholdContext(UUID householdId, UUID userId, String role) {
+
+    public boolean isPlanner() {
+        return "planner".equals(role);
+    }
+}

backend/src/main/java/com/recipeapp/common/ResourceNotFoundException.java

@@ -0,0 +1,7 @@
+package com.recipeapp.common;
+
+public class ResourceNotFoundException extends RuntimeException {
+    public ResourceNotFoundException(String message) {
+        super(message);
+    }
+}

backend/src/main/java/com/recipeapp/common/ValidationException.java

@@ -0,0 +1,7 @@
+package com.recipeapp.common;
+
+public class ValidationException extends RuntimeException {
+    public ValidationException(String message) {
+        super(message);
+    }
+}

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);
+    }
+}

backend/src/test/resources/application-test.yml

@@ -0,0 +1,12 @@
+spring:
+  datasource:
+    url: jdbc:tc:postgresql:16-alpine:///mealprep
+    username: mealprep
+    password: mealprep
+  jpa:
+    open-in-view: false
+    hibernate:
+      ddl-auto: validate
+  flyway:
+    enabled: true
+    locations: classpath:db/migration