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>

spring-angular-ui/src/app/components/_user-shared.scss

@@ -0,0 +1,51 @@
+// Общие стили пользователя: аватар с инициалами и чипы ролей (админка, профиль)
+
+.avatar {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  flex: none;
+  width: 36px;
+  height: 36px;
+  border-radius: 50%;
+  font: var(--mat-sys-title-small);
+  text-transform: uppercase;
+  background-color: var(--mat-sys-primary-container);
+  color: var(--mat-sys-on-primary-container);
+}
+
+.avatar.large {
+  width: 88px;
+  height: 88px;
+  font: var(--mat-sys-headline-medium);
+}
+
+.roles {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+}
+
+.role {
+  display: inline-block;
+  padding: 2px 10px;
+  border-radius: 8px;
+  font: var(--mat-sys-label-medium);
+  background-color: var(--mat-sys-surface-container-highest);
+  color: var(--mat-sys-on-surface-variant);
+}
+
+.role.ROLE_ADMIN {
+  background-color: var(--mat-sys-error-container);
+  color: var(--mat-sys-on-error-container);
+}
+
+.role.ROLE_MODERATOR {
+  background-color: var(--mat-sys-tertiary-container);
+  color: var(--mat-sys-on-tertiary-container);
+}
+
+.role.ROLE_USER {
+  background-color: var(--mat-sys-secondary-container);
+  color: var(--mat-sys-on-secondary-container);
+}