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/pom.xml

@@ -16,6 +16,7 @@
 	<properties>
 		<java.version>21</java.version>
 		<springdoc.version>3.0.2</springdoc.version>
+		<testcontainers.version>2.0.4</testcontainers.version>
 	</properties>
 	<dependencies>
 		<dependency>
@@ -64,6 +65,18 @@
 			<artifactId>spring-security-test</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>junit-jupiter</artifactId>
+			<version>${testcontainers.version}</version>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>postgresql</artifactId>
+			<version>${testcontainers.version}</version>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 
 	<build>