Claude AI (web) sends a Cognito-issued client_id with an HTTPS redirect
URI, but the client registry only supported localhost redirect URIs via
ALLOWED_MCP_CLIENTS. Add ALLOWED_MCP_CLOUD_CLIENTS env var for web-based
clients with format "client_id|redirect_uri".
Also fix the DCR proxy to return a clear error when the upstream IdP
(e.g. Cognito) doesn't support dynamic client registration, instead of
silently falling back to a Nextcloud-specific endpoint that fails.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
