feat: 3-tier role system across all platform apps

Portal:
- Replace is_admin boolean with role column (admin/advanced/standard)
- Settings UI: 3-tier portal role select + per-app role dropdowns
- /portal-return endpoint: re-establishes session from JWT for sub-app back-links
- /api/internal/nv-users: internal endpoint for NetworkView user sync
- portal/migrate_roles.py: one-time DB migration script
- Role badges (admin/advanced/standard) in topbar and settings table

DigiServer:
- Add editor and viewer roles (portal advanced->editor, standard->viewer)
- PlaylistPermission model: grant viewer users edit access to specific playlists
- app/utils/access.py: shared editor_required, admin_required, can_edit_playlist helpers
- Content routes: editor_required on create/delete, per-playlist permission check on mutations
- Admin: playlist_permissions route + template to manage viewer playlist grants
- Base template: hide Admin nav for viewers, Portal button (⬡) returns to portal
- content_list_new: hide create/delete for viewers; Manage vs View button per permission
- manage_playlist_content: view-only mode when user lacks edit permission

NetworkView:
- backend/src/middleware/rbac.js: requireRole + requireWriteAccess helpers
- site_permissions table: one site per advanced user
- All mutating routes guarded (admin=all, advanced=assigned site, viewer=read-only)
- Portal SSO auto-upsert: user row synced from X-Auth-Role on every request
- GET /api/users: merges portal users list with local NV data (all 4 portal users visible)
- GET/PUT /api/users/:id/site-permission: assign site to advanced user
- Settings Users tab: role badges, site dropdown for advanced, (portal only) indicator
- Sidebar: ⬡ Portal button between Settings and Logout
- Frontend build: VITE_API_BASE=/networkview/api now set in start-dev.sh

IT Assets / Server Monitor:
- portal_sso.py updated: map advanced->editor/viewer, standard->readonly
- AdminUser model: add editor role + is_editor property

digiserver-v2/app/templates/base.html

@@ -393,8 +393,17 @@
                         <img src="{{ url_for('static', filename='icons/playlist.svg') }}" alt="">
                         Playlists
                     </a>
+                    {% if current_user.role in ('admin', 'editor') %}
                     <a href="{{ url_for('admin.admin_panel') }}">Admin</a>
-                    <a href="{{ url_for('auth.logout') }}">Logout ({{ current_user.username }})</a>
+                    {% endif %}
+                    <a href="/portal-return" title="Back to Enterprise Portal Dashboard"
+                       style="background: rgba(255,255,255,0.15); border: 1px solid rgba(255,255,255,0.3);">
+                        ⬡ Portal
+                    </a>
+                    <a href="{{ url_for('auth.logout') }}" style="display:flex; flex-direction:column; align-items:center; line-height:1.2;">
+                        <span>Logout ({{ current_user.username }})</span>
+                        <span style="font-size:0.7em; opacity:0.7; text-transform:uppercase; letter-spacing:0.05em;">{{ current_user.role }}</span>
+                    </a>
                     <button class="dark-mode-toggle" onclick="toggleDarkMode()" title="Toggle Dark Mode">
                         <img id="theme-icon" src="{{ url_for('static', filename='icons/moon.svg') }}" alt="Toggle theme">
                     </button>