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/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