fix(reward-service): 降级 Prisma 版本 7.x → 5.22.0

修复 PrismaClient 初始化错误:
TypeError: Cannot read properties of undefined (reading '__internal')

Prisma 7.x 是预览版本，存在兼容性问题，降级到稳定的 5.22.0 版本。

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

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

backend/services/reward-service/package.json

@@ -30,7 +30,7 @@
     "@nestjs/platform-express": "^11.0.1",
     "@nestjs/schedule": "^6.0.1",
     "@nestjs/swagger": "^11.2.3",
-    "@prisma/client": "^7.0.1",
+    "@prisma/client": "^5.22.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.3",
     "dotenv": "^17.2.3",
@@ -60,7 +60,7 @@
     "globals": "^16.0.0",
     "jest": "^30.0.0",
     "prettier": "^3.4.2",
-    "prisma": "^7.0.1",
+    "prisma": "^5.22.0",
     "source-map-support": "^0.5.21",
     "supertest": "^7.0.0",
     "ts-jest": "^29.2.5",