feat: add custom OpenAI-compatible endpoint support for cloud agents

Users can now connect any OpenAI-compatible API gateway (e.g. New API,
One API, custom deployments) as a cloud agent by providing a base URL,
model name, and API key.

- Add base_url column to CloudAgentConfig + migration 019
- Add "Custom Endpoint" provider that accepts any model name
- Update cloud agent invocation to pass base_url to OpenAI client
- Frontend: custom provider shows URL + model name text inputs
- Auto-appends /v1 to base URL if not present