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/dto/AcceptInviteResponse.java

@@ -0,0 +1,9 @@
+package com.recipeapp.household.dto;
+
+import java.util.UUID;
+
+public record AcceptInviteResponse(
+        UUID householdId,
+        String householdName,
+        String role
+) {}