feat: finish demo — port demo-monolith cleanup, separate UI and API containers

Frontend
- Bring spring-angular-ui back under version control with the Angular 22 +
  Material UI from demo-monolith; stop committing the built static/ folder
- Serve the UI from nginx, which proxies /api to the backend so the JWT
  cookie works on a single origin, and falls back to index.html for routes

Backend
- Add PUT /api/users/{id}/roles (admin only) that keeps at least one admin
  and does not let an admin drop their own admin role
- Tighten security: read tutorials publicly, change them only when logged
  in, users/roles API for admins only; hide password hashes in JSON
- Remove unused Settings and Test endpoints, DTO mappers, commented code
- Flyway V2 aligns the schema with the entities; Hibernate now runs with
  ddl-auto: validate
- JWT lifetime matches the 24h cookie; fix the cookie name typo
- Keep CORS only for calling the API directly (ng serve without proxy)

Docker
- Backend Dockerfile: Gradle build + JRE runtime with layer caching,
  non-root user and healthcheck; add .dockerignore
- UI Dockerfile: Node 24 build + nginx with healthcheck
- docker-compose runs its own Postgres, starts services in health order
  and always rebuilds images

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,22 @@
+# Всё, что не нужно для сборки образа бэкенда: ускоряет передачу контекста
+# и не даёт попасть внутрь локальным секретам
+.git
+.gitignore
+.gitattributes
+.idea
+.vscode
+.gradle
+build
+out
+*.iml
+
+.env
+.env.*
+!.env.example
+
+Dockerfile
+docker-compose*.yml
+.dockerignore
+
+# Фронт собирается в своём образе (контекст — spring-angular-ui)
+spring-angular-ui