Implement household domain with outside-in TDD (15 tests)

Controller (5 tests): create household, get mine, get members,
create invite, accept invite.

Service (10 tests): household creation with planner role + seed
data (categories, tags, staple ingredients), conflict when already
in household, invite code generation with 48h expiry, accept invite
with expired/used/conflict validation.

Also includes:
- Household, HouseholdMember, HouseholdInvite JPA entities
- HouseholdInvite repository with findByInviteCode
- Ingredient, IngredientCategory, Tag entities + repositories
  (created early for seed data, will be extended in recipe domain)
- Fixed BackendApplicationTests to use AbstractIntegrationTest

Total: 38 tests passing.

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

backend/src/main/java/com/recipeapp/household/HouseholdMemberRepository.java

@@ -3,9 +3,11 @@ package com.recipeapp.household;
 import com.recipeapp.household.entity.HouseholdMember;
 import org.springframework.data.jpa.repository.JpaRepository;
 
+import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
 public interface HouseholdMemberRepository extends JpaRepository<HouseholdMember, UUID> {
     Optional<HouseholdMember> findByUserEmailIgnoreCase(String email);
+    List<HouseholdMember> findByHouseholdId(UUID householdId);
 }