Upstream API passthrough

Control which /upstream model paths are ignored instead of triggering a model swap.

upstream controls the /upstream/<model>/<path> passthrough endpoint. Its ignorePaths expressions prevent static or UI paths from triggering a model swap; it does not configure a fallback OpenAI service for unmatched requests.

Use RE2-compatible expressions and keep the default static-asset patterns unless a client needs different paths. Check logs to distinguish an ignored path from a model-routing request.