fix(backup-service): 添加 DATABASE_URL 用于 Prisma generate

构建时需要 dummy DATABASE_URL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/services/backup-service/Dockerfile

@@ -13,8 +13,8 @@ RUN npm ci
 # Copy source code
 COPY . .
 
-# Generate Prisma client
-RUN npx prisma generate
+# Generate Prisma client (dummy DATABASE_URL for build time only)
+RUN DATABASE_URL="postgresql://user:pass@localhost:5432/db" npx prisma generate
 
 # Build the application
 RUN npm run build