feat: complete server-backed six-lesson app and trial tooling

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.docker.example

@@ -3,10 +3,14 @@
 # Build-time: API base URL for the Vue app (/api = nginx proxies to backend).
 VITE_API_URL=/api
 
-# Host port for the frontend container
+# Host port published for the frontend container
 FRONTEND_PORT=8080
 
-# Backend env (used by ../backend-nest service in compose)
-JWT_SECRET=replace-with-a-long-random-secret
+# Backend env (compose service `backend`)
+JWT_SECRET=<replace-with-48+-character-random-secret>
 # Must match the public URL users open in the browser (scheme + host + port).
-CORS_ORIGIN=http://your-server:8080
+CORS_ORIGIN=http://<your-trial-host>:8080
+
+# Backend (set in compose for trial stack — override only if needed)
+# TRUST_PROXY=true
+# TRUST_PROXY_CIDRS=10.0.0.0/8,172.16.0.0/12,192.168.0.0/16