[Docs]: Role-Based OpenAPI Documentation Visibility

### Is your feature request related to a problem?

Admin-only endpoints (e.g., `/users`, `/tokens`) currently appear in the default OpenAPI schema and Swagger UI for all authenticated users. While route-level authorization prevents access, exposing these endpoints in public docs creates unnecessary visibility and potential confusion.

### Describe the solution you'd like

- [ ] Tag admin-only endpoints (e.g., `tags=["admin"]`)
- [ ] Disable default `docs_url` and `openapi_url`
- [ ] Implement custom `/openapi.json` generation
- [ ] Filter out `admin`-tagged endpoints (e.g., `/users`, `/tokens`) for non-admin users
- [ ] Serve `/docs` behind authentication
- [ ] Maintain route-level authorization enforcement

### Describe alternatives you've considered

- [ ] Using `include_in_schema=False` (too static; hides endpoints for admins as well)
- [ ] Maintaining separate admin documentation service (adds complexity)

### Additional context

This change affects documentation visibility only and does not modify endpoint authorization behavior.