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/models/user.py

@@ -24,7 +24,8 @@ class User(db.Model, UserMixin):
     id = db.Column(db.Integer, primary_key=True)
     username = db.Column(db.String(80), unique=True, nullable=False, index=True)
     password = db.Column(db.String(120), nullable=False)
-    role = db.Column(db.String(20), nullable=False, default='user', index=True)
+    role = db.Column(db.String(20), nullable=False, default='viewer', index=True)
+    # Valid roles: 'admin' | 'editor' | 'viewer'
     theme = db.Column(db.String(20), default='light')
     created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
     last_login = db.Column(db.DateTime, nullable=True)
@@ -37,6 +38,16 @@ class User(db.Model, UserMixin):
     def is_admin(self) -> bool:
         """Check if user has admin role."""
         return self.role == 'admin'
+
+    @property
+    def is_editor(self) -> bool:
+        """True for admin and editor roles (can manage content)."""
+        return self.role in ('admin', 'editor')
+
+    @property
+    def is_viewer(self) -> bool:
+        """True for all authenticated users (read access)."""
+        return True
     
     def update_last_login(self) -> None:
         """Update last login timestamp."""