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/home/home.component.scss

@@ -0,0 +1,49 @@
+@use '../tutorial-shared';
+
+.grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
+  gap: 16px;
+}
+
+.tutorial-card {
+  height: 100%;
+}
+
+.card-icon {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background-color: var(--mat-sys-primary-container);
+  color: var(--mat-sys-on-primary-container);
+}
+
+mat-card-content {
+  padding-top: 12px;
+}
+
+.description {
+  margin: 0;
+  white-space: pre-line;
+  color: var(--mat-sys-on-surface-variant);
+}
+
+.demo-banner {
+  display: flex;
+  align-items: flex-start;
+  gap: 12px;
+  margin-bottom: 24px;
+  padding: 16px;
+  border-radius: 12px;
+  font: var(--mat-sys-body-medium);
+  background-color: var(--mat-sys-secondary-container);
+  color: var(--mat-sys-on-secondary-container);
+
+  code {
+    padding: 1px 6px;
+    border-radius: 4px;
+    font-weight: 500;
+    background-color: var(--mat-sys-surface);
+    color: var(--mat-sys-primary);
+  }
+}