fix(members): guard against removing the last planner from household

removeMember now checks the planner count before deleting a planner
member. Throws ConflictException("Cannot remove the last planner")
when only one planner remains, matching the spec requirement in S4.

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

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

@@ -134,6 +134,13 @@ public class HouseholdService {
             throw new ConflictException("Planner cannot remove yourself");
         }
 
+        if ("planner".equals(target.getRole())) {
+            long plannerCount = householdMemberRepository.countByHouseholdIdAndRole(householdId, "planner");
+            if (plannerCount <= 1) {
+                throw new ConflictException("Cannot remove the last planner");
+            }
+        }
+
         householdMemberRepository.delete(target);
     }
 

backend/src/test/java/com/recipeapp/household/HouseholdServiceTest.java

@@ -348,6 +348,24 @@ class HouseholdServiceTest {
                 .hasMessageContaining("cannot remove yourself");
     }
 
+    @Test
+    void removeMemberShouldThrow409WhenRemovingLastPlanner() {
+        var planner = testUser();
+        var target = new UserAccount("tom@example.com", "Tom", "hashed");
+        var household = new Household("Smith family", planner);
+        var plannerMembership = new HouseholdMember(household, planner, "planner");
+        var targetMembership = new HouseholdMember(household, target, "planner");
+        var targetId = UUID.randomUUID();
+
+        when(householdMemberRepository.findByUserEmailIgnoreCase("sarah@example.com")).thenReturn(Optional.of(plannerMembership));
+        when(householdMemberRepository.findByHouseholdIdAndUserId(any(), eq(targetId))).thenReturn(Optional.of(targetMembership));
+        when(householdMemberRepository.countByHouseholdIdAndRole(any(), eq("planner"))).thenReturn(1L);
+
+        assertThatThrownBy(() -> householdService.removeMember("sarah@example.com", targetId))
+                .isInstanceOf(ConflictException.class)
+                .hasMessageContaining("last planner");
+    }
+
     @Test
     void removeMemberShouldThrow404WhenTargetNotInHousehold() {
         var planner = testUser();