Swagger/UI Freshness

[mockodin]

Greetings

I'm curious about SwaggerUI freshness - I see today the files needed are vendored, is that a hard requirement? I believe we could benefit from leveraging an intermediary module like flask-swagger-ui which provides regular automated updates.

Motivation

Flask-Rebar currently owns a copied Swagger UI distribution and a local HTML
template. This has several costs:

• Updating Swagger UI requires manually replacing generated frontend files.
• Flask-Rebar duplicates resources already maintained by a dedicated package.
• Large minified assets obscure meaningful Python changes during review.
• Applications cannot configure common Swagger UI or OAuth settings per
  registry.
• The copied UI bundle can age independently of Flask-Rebar's Python code.

Goals

• Serve Swagger UI from a declared Python dependency (pinnable / user-overridable)
• Remove Flask-Rebar's duplicate JavaScript, CSS, source maps, icons, OAuth
  resources, and HTML template.
• Preserve existing public routes, endpoints, and Python call signatures.
• Support JSON-serializable Swagger UI settings per handler registry.
• Support OAuth initialization settings per handler registry.
• Keep registry configuration isolated from caller mutation and cloned
  registries.
• Render configuration safely inside HTML script contexts.
• Keep generated Swagger/OpenAPI documents unchanged.

I successfully wired-up flask-swagger-ui==5.32.14 locally to act the source for swagger UI static files. Even now we don't leverage all swagger UI features, but this would give is some modernization by default. Notably Dark Mode and accessibility enhancements.

If there is interest, I can roll this into a PR.