fix: F12 P2 follow-up — batch imports set departmentId, optimize getDescendantIds, enforce campus root invariant

apps/server/src/students/students.service.ts

@@ -115,6 +115,7 @@ export class StudentsService {
       supervisor?: string;
       tenantId?: number;
     }[],
+    departmentId?: number,
   ) {
     let imported = 0;
     let skipped = 0;
@@ -140,6 +141,7 @@ export class StudentsService {
           organization: row.organization || undefined,
           supervisor: row.supervisor || undefined,
           tenantId: row.tenantId || undefined,
+          departmentId: departmentId ?? undefined,
         }),
       );
       imported++;