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/integration/dingtalk.service.ts

@@ -663,12 +663,13 @@ export class DingTalkService {
       }
     }
 
-    // Create mapping
+    // ponytail: find student by userId — method rewritten in Task 2
+    const student = await this.studentRepo.findOne({ where: { userId: user.id } });
+    if (!student) return;
     mapping = this.studentDingMappingRepo.create({
       dingUserId: du.userid,
-      studentId: user.id,
+      studentId: student.id,
     });
-    await this.studentDingMappingRepo.save(mapping);
   }
 
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