Add OpenAPI 3.1 specification for ComfyUI API

Adds a comprehensive OpenAPI 3.1 spec documenting all HTTP endpoints
exposed by ComfyUI's server, including prompt execution, queue management,
file uploads, userdata, settings, system stats, object info, assets,
and internal routes.

The spec was validated against the source code with adversarial review
from multiple models, and passes Spectral linting with zero errors.

Also removes openapi.yaml from .gitignore so the spec is tracked.

.gitignore

@@ -21,6 +21,5 @@ venv*/
 *.log
 web_custom_versions/
 .DS_Store
-openapi.yaml
 filtered-openapi.yaml
 uv.lock