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

src/auth/external-verifier.ts

@@ -56,7 +56,7 @@ export class ExternalAuthVerifier implements OAuthTokenVerifier {
     try {
       // Token introspection is OAuth 2.0 standard (RFC 7662) for validating tokens
       // The auth server checks if the token is valid and returns metadata about it
-      const response = await fetch(`${this.authServerUrl}/oauth/introspect`, {
+      const response = await fetch(`${this.authServerUrl}/introspect`, {
         method: 'POST',
         headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
         body: `token=${encodeURIComponent(token)}`,