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/java/com/recipeapp/household/HouseholdService.java

@@ -17,6 +17,7 @@ import com.recipeapp.recipe.TagRepository;
 import com.recipeapp.recipe.entity.Ingredient;
 import com.recipeapp.recipe.entity.IngredientCategory;
 import com.recipeapp.recipe.entity.Tag;
+import org.springframework.beans.factory.annotation.Value;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
@@ -38,6 +39,9 @@ public class HouseholdService {
     private final TagRepository tagRepository;
     private final VarietyScoreConfigRepository varietyScoreConfigRepository;
 
+    @Value("${app.base-url}")
+    private String baseUrl;
+
     private static final SecureRandom RANDOM = new SecureRandom();
     private static final String CODE_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
 
@@ -262,7 +266,7 @@ public class HouseholdService {
     private InviteResponse toInviteResponse(HouseholdInvite invite) {
         return new InviteResponse(
                 invite.getInviteCode(),
-                "https://yourapp.com/join/" + invite.getInviteCode(),
+                baseUrl + "/join/" + invite.getInviteCode(),
                 invite.getExpiresAt());
     }
 }