refactor: 移除 SQLite 支持，仅保留 MySQL

apps/server/src/attendance/attendance-lesson.service.ts

@@ -244,8 +244,8 @@ export class AttendanceLessonService {
       } catch (err: unknown) {
         const code = (err as Record<string, unknown>).code;
         const errno = (err as Record<string, unknown>).errno;
-        // MySQL: ER_DUP_ENTRY or errno 1062; SQLite: SQLITE_CONSTRAINT
-        if (code === 'ER_DUP_ENTRY' || errno === 1062 || code === 'SQLITE_CONSTRAINT') {
+        // MySQL: ER_DUP_ENTRY or errno 1062
+        if (code === 'ER_DUP_ENTRY' || errno === 1062) {
           const existing = await sessionRepo.findOne({
             where: { scheduleId, lessonDate },
           });