feat(members): implement DELETE/PATCH member + GET invites backend endpoints

- Add V006 migration: invalidated_at column + partial unique index on household_invite
- Add findByHouseholdIdAndInvalidatedAtIsNull, findByHouseholdIdAndUserId, countByHouseholdIdAndRole
- Add ChangeRoleRequest DTO
- HouseholdService: getActiveInvite, createInvite (regenerate), removeMember, changeMemberRole
- HouseholdController: GET /v1/households/mine/invites, DELETE/PATCH /v1/households/mine/members/{userId}

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

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

@@ -23,6 +23,8 @@ import org.springframework.transaction.annotation.Transactional;
 import java.security.SecureRandom;
 import java.time.Instant;
 import java.util.List;
+import java.util.Optional;
+import java.util.UUID;
 
 @Service
 public class HouseholdService {
@@ -91,21 +93,73 @@ public class HouseholdService {
                 .toList();
     }
 
+    @Transactional
+    public MemberResponse changeMemberRole(String requesterEmail, UUID targetUserId, String newRole) {
+        HouseholdMember requester = findMembership(requesterEmail);
+        UUID householdId = requester.getHousehold().getId();
+
+        HouseholdMember target = householdMemberRepository
+                .findByHouseholdIdAndUserId(householdId, targetUserId)
+                .orElseThrow(() -> new ResourceNotFoundException("Member not found in this household"));
+
+        if (target.getRole().equals(newRole)) {
+            return toMemberResponse(target);
+        }
+
+        if ("member".equals(newRole) && "planner".equals(target.getRole())) {
+            long plannerCount = householdMemberRepository.countByHouseholdIdAndRole(householdId, "planner");
+            if (plannerCount <= 1) {
+                throw new ConflictException("Cannot degrade the last planner");
+            }
+        }
+
+        target.setRole(newRole);
+        return toMemberResponse(householdMemberRepository.save(target));
+    }
+
+    @Transactional
+    public void removeMember(String requesterEmail, UUID targetUserId) {
+        HouseholdMember requester = findMembership(requesterEmail);
+        UUID householdId = requester.getHousehold().getId();
+
+        HouseholdMember target = householdMemberRepository
+                .findByHouseholdIdAndUserId(householdId, targetUserId)
+                .orElseThrow(() -> new ResourceNotFoundException("Member not found in this household"));
+
+        if (target.getUser().getEmail().equalsIgnoreCase(requesterEmail)) {
+            throw new ConflictException("Planner cannot remove yourself");
+        }
+
+        householdMemberRepository.delete(target);
+    }
+
+    @Transactional(readOnly = true)
+    public Optional<InviteResponse> getActiveInvite(String userEmail) {
+        HouseholdMember member = findMembership(userEmail);
+        return householdInviteRepository
+                .findByHouseholdIdAndInvalidatedAtIsNull(member.getHousehold().getId())
+                .filter(invite -> invite.getExpiresAt().isAfter(Instant.now()))
+                .map(this::toInviteResponse);
+    }
+
     @Transactional
     public InviteResponse createInvite(String userEmail) {
         HouseholdMember member = findMembership(userEmail);
         Household household = member.getHousehold();
 
+        householdInviteRepository.findByHouseholdIdAndInvalidatedAtIsNull(household.getId())
+                .ifPresent(existing -> {
+                    existing.setInvalidatedAt(Instant.now());
+                    householdInviteRepository.save(existing);
+                });
+
         String code = generateInviteCode();
         Instant expiresAt = Instant.now().plusSeconds(48 * 3600);
 
         HouseholdInvite invite = householdInviteRepository.save(
                 new HouseholdInvite(household, code, expiresAt));
 
-        return new InviteResponse(
-                invite.getInviteCode(),
-                "https://yourapp.com/join/" + invite.getInviteCode(),
-                invite.getExpiresAt());
+        return toInviteResponse(invite);
     }
 
     @Transactional
@@ -204,4 +258,11 @@ public class HouseholdService {
                 member.getRole(),
                 member.getJoinedAt());
     }
+
+    private InviteResponse toInviteResponse(HouseholdInvite invite) {
+        return new InviteResponse(
+                invite.getInviteCode(),
+                "https://yourapp.com/join/" + invite.getInviteCode(),
+                invite.getExpiresAt());
+    }
 }