feat(config): add ENDORSER_URL for upcoming Endorser auth

Expose the Endorser API base URL from the environment (defaulting to production) so later auth work can call it without hardcoding.

.env.example

@@ -1,6 +1,9 @@
 # HTTP port (default: 3003)
 PORT=3003
 
+# Endorser API base URL (default: https://api.endorser.ch)
+# ENDORSER_URL=https://api.endorser.ch
+
 # Firebase Admin: inline service account JSON (one line).
 # If unset, uses Application Default Credentials (e.g. GOOGLE_APPLICATION_CREDENTIALS).
 # FIREBASE_SERVICE_ACCOUNT_JSON={"type":"service_account",...}