feat(portal-self-service): runtime config + nginx serve/proxy image (refs #68)

The app loads /config.json at startup (main.ts) so the OIDC authority is set per
environment from one build; appConfig becomes a factory and derives redirectUrl +
secureApiOrigin from the app origin (same-origin as the BFF). A multi-stage
Dockerfile builds the app and serves it via nginx, reverse-proxying /self-service
+ /openbaar to the bff (relative URLs → no CORS); nginx resolves the BFF at request
time. The compose image bakes config.json with the keycloak:8080 authority so the
browser's token issuer matches the BFF (ADR-0010).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/self-service/public/config.json

@@ -0,0 +1,3 @@
+{
+  "authority": "http://localhost:8180/realms/digid"
+}