Fix OAuth routing: send /oauth/* to PDS, not frontend
The IngressRoute was sending all /oauth/ paths to the frontend nginx,
which only serves static SPA files. OAuth protocol endpoints (PAR,
authorize, token, consent, etc.) need to reach the PDS backend.
Only /oauth/client-metadata.json stays on the frontend for hostname
substitution via sub_filter. All other /oauth/* falls through to the
PDS catch-all route.