fix: correct studentId-as-userId bugs in 3 files

- dingtalk.service.ts syncOneUser: find Student by userId before mapping,
  use Student.id (not User.id) as studentId FK
- sync.service.ts importDingTalkUsers: type coercion for studentId
  (method deleted in Task 4, minimal compile fix)
- schedule-sync.service.ts getStatus: remove broken teacher mapping
  query (teacher scheduling deprecated); hardcode mappedTeachers=0,
  totalTeachers=0

apps/server/src/sync/sync.service.ts

@@ -260,7 +260,7 @@ export class SyncService {
           if (!existingMappingForUser) {
             const mapping = manager.create(StudentDingMapping, {
               dingUserId: u.dingUserId,
-              studentId: user.id,
+              studentId: (user as any).id,
             });
             await manager.save(mapping);
           }