fix: resolve 5 review findings

Critical 1: Remove broken '钉钉绑定' tab (IntegConfig) — deleted /rbac/user-ding-mappings calls
Critical 2: Fix decorator placement in CreateClassDto — @IsOptional @IsArray restored to teachers
Important 1: Restore sync.service.spec.ts from 1fa3363, strip importDingTalkUsers tests
Important 2: Fix N+1 in batchImportStudents — batched find/save with In() operator
Important 3: Add migrate-student-ding-mapping.sql

apps/server/migrate-student-ding-mapping.sql

@@ -0,0 +1,14 @@
+-- Migration: Replace user_ding_mapping with student_ding_mapping
+-- Date: 2026-07-09
+
+-- Drop old table
+DROP TABLE IF EXISTS user_ding_mapping;
+
+-- Create new table
+CREATE TABLE IF NOT EXISTS student_ding_mapping (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  ding_user_id VARCHAR(100) NOT NULL UNIQUE,
+  student_id INTEGER NOT NULL UNIQUE,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+  FOREIGN KEY (student_id) REFERENCES students(id) ON DELETE CASCADE
+);