fix(portal-self-service): attach the DigiD token to relative BFF calls (refs #68)

After login the submit silently did nothing: the confirmation ("...is
ontvangen...") never rendered because the POST to the BFF went out with no
Authorization header, so the BFF rejected it and the no-error-handler
subscribe left the page unchanged.

Root cause: angular-auth-oidc-client's interceptor attaches the token when
`req.url.startsWith(secureRoute)`. The api-client calls the BFF with RELATIVE
URLs (same-origin via the nginx proxy), so `req.url` is `/self-service/...` —
but secureRoutes was configured as the app ORIGIN (`http://self-service`),
which a relative URL never starts with. No match → no token.

Configure secureRoutes with the relative `/self-service/` prefix instead. The
unit test mocked the api-client, so only the walking-skeleton e2e exercises the
real token attachment — now green.

Verified against a focused stack (keycloak + self-service + real BFF + stub
domain): the submit now carries the bearer token, the BFF forwards to the
domain, and the portal shows the confirmation with the returned reference.

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

libs/auth/src/lib/digid-auth.providers.ts

@@ -13,8 +13,13 @@ export interface DigiadAuthOptions {
   authority: string;
   /** Where Keycloak redirects back to after login (usually the app origin). */
   redirectUrl: string;
-  /** The BFF origin whose requests get the bearer token attached (secure route). */
-  secureApiOrigin: string;
+  /**
+   * Route prefixes whose requests get the bearer token attached. The api-client calls the BFF with
+   * **relative** URLs (same-origin via the nginx proxy), so these must be relative path prefixes
+   * (e.g. `/self-service/`) — angular-auth-oidc-client matches `req.url.startsWith(route)`, and a
+   * relative `req.url` never starts with an absolute origin.
+   */
+  secureRoutes: string[];
 }
 
 /**
@@ -35,7 +40,7 @@ export function provideDigiadAuth(options: DigiadAuthOptions): EnvironmentProvid
           responseType: 'code',
           silentRenew: true,
           useRefreshToken: true,
-          secureRoutes: [options.secureApiOrigin],
+          secureRoutes: options.secureRoutes,
           logLevel: LogLevel.Warn,
         },
       },