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/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()));
+    }
+}