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/HouseholdController.java

@@ -0,0 +1,56 @@
+package com.recipeapp.household;
+
+import com.recipeapp.common.ApiResponse;
+import com.recipeapp.household.dto.*;
+import jakarta.validation.Valid;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+import java.security.Principal;
+import java.util.List;
+
+@RestController
+@RequestMapping("/v1")
+public class HouseholdController {
+
+    private final HouseholdService householdService;
+
+    public HouseholdController(HouseholdService householdService) {
+        this.householdService = householdService;
+    }
+
+    @PostMapping("/households")
+    public ResponseEntity<ApiResponse<HouseholdResponse>> createHousehold(
+            Principal principal,
+            @Valid @RequestBody CreateHouseholdRequest request) {
+        HouseholdResponse response = householdService.createHousehold(principal.getName(), request);
+        return ResponseEntity.status(HttpStatus.CREATED).body(ApiResponse.success(response));
+    }
+
+    @GetMapping("/households/mine")
+    public ResponseEntity<ApiResponse<HouseholdResponse>> getMyHousehold(Principal principal) {
+        HouseholdResponse response = householdService.getMyHousehold(principal.getName());
+        return ResponseEntity.ok(ApiResponse.success(response));
+    }
+
+    @GetMapping("/households/mine/members")
+    public ResponseEntity<List<MemberResponse>> getMembers(Principal principal) {
+        List<MemberResponse> members = householdService.getMembers(principal.getName());
+        return ResponseEntity.ok(members);
+    }
+
+    @PostMapping("/households/mine/invites")
+    public ResponseEntity<ApiResponse<InviteResponse>> createInvite(Principal principal) {
+        InviteResponse response = householdService.createInvite(principal.getName());
+        return ResponseEntity.status(HttpStatus.CREATED).body(ApiResponse.success(response));
+    }
+
+    @PostMapping("/invites/{code}/accept")
+    public ResponseEntity<ApiResponse<AcceptInviteResponse>> acceptInvite(
+            Principal principal,
+            @PathVariable String code) {
+        AcceptInviteResponse response = householdService.acceptInvite(principal.getName(), code);
+        return ResponseEntity.ok(ApiResponse.success(response));
+    }
+}