fix: retrieve discovery document in validate_token function for improved token validation

backend/app/auth.py

@@ -82,6 +82,7 @@ async def validate_token(credentials: HTTPAuthorizationCredentials = Security(se
     token = credentials.credentials
     key = get_signing_key(token)
     public_key = jwk.construct(key)
+    discovery = get_discovery()
     try:
         # Only pass `audience` to the decoder if configured. Some providers
         # (or local development setups) may not include the aud claim in a