feat(backend): make invite share URL base configurable via app.base-url

Replaces hardcoded \"https://yourapp.com\" with a Spring property.

- application.yml: app.base-url defaults to http://localhost:5173
- application-docker.yml: reads APP_BASE_URL env var, same default
- HouseholdService: injects @Value("${app.base-url}") and uses it in
  toInviteResponse() to build shareUrl
- HouseholdServiceTest: sets field via ReflectionTestUtils in @BeforeEach;
  adds test asserting shareUrl starts with configured base URL

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/application.yml

@@ -30,3 +30,6 @@ spring:
 
 server:
   port: 8080
+
+app:
+  base-url: http://localhost:5173