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);
+    }
+}