feat: indent echo responses for readable evidence

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/main/resources/application.yml

@@ -9,6 +9,11 @@ server:
 spring:
   application:
     name: keycloak-pattern-api
+  jackson:
+    serialization:
+      # /api/echo is read by humans and captured as evidence screenshots, so the
+      # response is indented rather than relying on a browser's JSON viewer.
+      indent-output: true
   security:
     oauth2:
       resourceserver: