Add optional custom base URL for AI providers

[johncarbon88]

Add optional custom base URL for AI providers

[seriaati]

A couple of things to change:

1. To avoid SSRF, validate that the base_url is not something suspicious, like an internal service.
2. Add an env var flag for disabling this feature, I don't want to enable it on the hosted instance due to possible risks. You can however, use it freely on the self-hosted version.
3. In _factory.py, there is now base_url=base_url or "https://openrouter.ai/api/v1" for OpenRouter. I don't think this is needed, because OpenRouter obviously is OpenAI-compatible and there is no need to set a base URL for OpenRouter. This can also confuse the user if they set both a custom base URL + select OpenRouter.
4. base_url.strip() normalization is duplicated across different files. Would probably easier to just use a Pydantic schema validator and do it there.

[seriaati]

5. The PR's migration down revision needs to be updated to the latest one.