chore(deploy): set production LOG_LEVEL to error

Configure production deployments to use LOG_LEVEL=error for minimal log volume.

- Add LOG_LEVEL=error to docker-compose.yml environment

- Add LOG_LEVEL: error to K8s ConfigMap

- Update .env.example with dev/prod LOG_LEVEL guidance

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

k8s/gitea-assistant.yaml

@@ -10,6 +10,7 @@ metadata:
     app.kubernetes.io/part-of: gitea-assistant
 data:
   PORT: "5174"
+  LOG_LEVEL: "error"
   # All settings (Gitea connection, webhook secret, admin password, review engine,
   # Feishu, memory, etc.) are managed through the Admin Dashboard Web UI.
   # They are auto-seeded with secure defaults on first boot.