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

portal/app/static/css/portal.css

@@ -105,12 +105,13 @@ code { font-family: 'SFMono-Regular', Consolas, monospace; font-size: 0.85em; }
   font-size: 0.7rem;
   padding: 2px 6px;
   border-radius: 20px;
-  background: rgba(56,139,253,0.2);
-  color: var(--accent-blue);
   font-weight: 600;
   text-transform: uppercase;
   letter-spacing: 0.04em;
 }
+.role-tag--admin    { background: rgba(56,139,253,0.2);  color: var(--accent-blue); }
+.role-tag--advanced { background: rgba(245,158,11,0.2);  color: #f59e0b; }
+.role-tag--standard { background: rgba(110,118,129,0.15); color: var(--text-secondary); }
 
 .btn-logout {
   color: var(--text-muted);
@@ -376,6 +377,7 @@ code { font-family: 'SFMono-Regular', Consolas, monospace; font-size: 0.85em; }
   letter-spacing: 0.04em;
 }
 .badge-admin    { background: rgba(56,139,253,0.2);  color: var(--accent-blue); }
+.badge-advanced { background: rgba(245,158,11,0.2);  color: #f59e0b; }
 .badge-user     { background: rgba(110,118,129,0.2); color: var(--text-secondary); }
 .badge-active   { background: rgba(63,185,80,0.2);  color: var(--accent-green); }
 .badge-inactive { background: rgba(110,118,129,0.2); color: var(--text-muted); }