fix(backup-service): 移除 schema.prisma 中的 url (Prisma 7 配置在 prisma.config.ts)

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

backend/services/backup-service/prisma/schema.prisma

@@ -6,7 +6,6 @@ generator client {
 
 datasource db {
   provider = "postgresql"
-  url      = env("DATABASE_URL")
 }
 
 // 备份分片存储