fix(backup-service): 添加 datasource url 到 Prisma schema

Prisma migrate/push 需要 datasource 中包含 url 属性。

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

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

@@ -6,6 +6,7 @@ generator client {
 
 datasource db {
   provider = "postgresql"
+  url      = env("DATABASE_URL")
 }
 
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