fix(auth): 修复 CDC consumer 类型错误

使用 ?? 运算符正确处理可选字段：
- update 使用 undefined 保持字段不变
- create 使用 null 明确设置为空值

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/auth-service/src/infrastructure/messaging/cdc/legacy-user-cdc.consumer.ts

@@ -137,8 +137,8 @@ export class LegacyUserCdcConsumer implements OnModuleInit, OnModuleDestroy {
       await this.prisma.syncedLegacyUser.upsert({
         where: { legacyId: BigInt(user.user_id) },
         update: {
-          phone: user.phone_number || null,
-          passwordHash: user.password_hash || null,
+          phone: user.phone_number ?? undefined,
+          passwordHash: user.password_hash ?? undefined,
           accountSequence: user.account_sequence,
           status: user.status,
           sourceSequenceNum: sequenceNum,
@@ -146,8 +146,8 @@ export class LegacyUserCdcConsumer implements OnModuleInit, OnModuleDestroy {
         },
         create: {
           legacyId: BigInt(user.user_id),
-          phone: user.phone_number || null,
-          passwordHash: user.password_hash || null,
+          phone: user.phone_number ?? null,
+          passwordHash: user.password_hash ?? null,
           accountSequence: user.account_sequence,
           status: user.status,
           legacyCreatedAt: new Date(user.registered_at),