fab: centralize FastAPI auth manager routing

[henry3260]

What

• Added shared root routers for FAB auth manager FastAPI routes.
• login, users, and roles now register endpoints via common routers with a single
  source of truth for prefix/tags.

Why

• Avoids duplicated prefix/tag definitions across route modules.
• Makes it easier to change FAB API prefix/tags in one place and reduces the risk of
  drift.

related: #57480 (comment)

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[henry3260]

HI @vincbeck , It looks like the FastAPI router isn't registering the endpoints correctly, which is why the generated OpenAPI spec is empty. I suspect we just need to import and include the login, roles, and users sub-routers into the main FAB router. WDYT?

[vincbeck]

There seems to be a lot of errors, let's try to fix them one by one