Fix endpoint consistency and documentation accuracy

- Remove /oauth prefix from introspect endpoint to match SDK pattern
- Fix auth-server endpoints: /introspect not /oauth/introspect
- Update external-verifier to call correct introspect endpoint
- Add 30-day expiry to client registration (REDIS_EXPIRY_TIMES.CLIENT_REGISTRATION)
- Remove inaccurate scratch/ directory references from README
- Add backlinks from auth-server/README.md to main README sections
- Verified all implementation details against running servers and Redis data

Author: bhosmer-ant