fix(grafana): sync Loki datasource settings

[singchia]

Summary

• sync the Grafana Loki datasource from system_settings.loki during Grafana Sync
• keep the legacy datasource field while returning all synced datasources
• mark provisioned Loki datasources editable so Grafana API updates are allowed

Tests

• go test ./internal/manager/biz/grafana

Fixes #122

Author confirmation

• I have read and agree to the project contribution guide in CONTRIBUTING.md.

[singchia]

补充了一轮 #122 修复，已推送 commit d582e2b：

• 新增只同步 Loki datasource 的 POST /v1/integrations/grafana/sync-loki；
• 保存 Loki 设置后自动调用该接口；
• 不再因保存 Loki 配置而重新同步 Prometheus datasource 或覆盖 dashboard；
• Grafana 未配置或同步失败时，Loki 设置仍保留，页面显示明确警告；
• 增加接口鉴权/调用测试。

验证：

• go test ./internal/manager/biz/grafana ./internal/manager/server/integration
• go test ./internal/manager/...

均通过。