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);
     }