feat: add multi-administrator support with RBAC and fix backup/restore for S3

## Multi-Administrator System
- Add role-based access control (RBAC) with predefined roles (Super Admin, Admin, Editor, Viewer)
- Add granular permissions system for all admin operations
- Add admin user management page with invite functionality
- Add email invitation system for new administrators
- Add permission middleware protecting all admin routes
- Add PermissionGate component for frontend permission checks
- Track event creator (created_by) for audit purposes

## Backup & Restore Fixes
- Fix S3 backup: endpoint URL handling, manifest loading, field name compatibility
- Fix S3 restore: add list-backups endpoint, transform S3 config from frontend format
- Fix PostgreSQL compatibility: add .returning('id') for insert operations
- Fix disk space check: use df command, handle unknown space gracefully
- Fix dry-run validation to not block on warnings
- Fix req.user → req.admin in restore routes

## Database Migrations
- 054: Add roles table with predefined roles
- 055: Add permissions table
- 056: Add role_permissions junction table
- 057: Add role_id to admin_users
- 058: Add admin_invitations table
- 059: Add admin email templates
- 060: Add created_by to events table

## Other Improvements
- Update .gitignore to exclude planning docs and local backup directory
- Remove SQLite database file from tracking
- Add i18n translations for user management (EN/DE)

frontend/src/App.tsx

@@ -10,9 +10,9 @@ import { ThemeProvider } from './contexts/ThemeContext';
 import { GalleryPage } from './pages/GalleryPage';
 import { PreviewPage } from './pages/gallery/PreviewPage';
 import { LegalPage } from './pages/public/LegalPage';
-import { 
-  AdminLoginPage, 
-  AdminDashboard, 
+import {
+  AdminLoginPage,
+  AdminDashboard,
   EventsListPage,
   CreateEventPage,
   EventDetailsPage,
@@ -23,8 +23,10 @@ import {
   BrandingPage,
   SettingsPage,
   BackupManagement,
-  CMSPage
+  CMSPage,
+  UserManagementPage
 } from './pages/admin';
+import { AcceptInvitePage } from './pages/public/AcceptInvitePage';
 import { AdminLayout, AdminAuthWrapper } from './components/admin';
 import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
@@ -128,10 +130,14 @@ function App() {
                       <Route path="settings" element={<SettingsPage />} />
                       <Route path="backup" element={<BackupManagement />} />
                       <Route path="cms" element={<CMSPage />} />
+                      <Route path="users" element={<UserManagementPage />} />
                       <Route index element={<Navigate to="/admin/dashboard" replace />} />
                     </Route>
                   </Route>
 
+                  {/* Public invitation acceptance page */}
+                  <Route path="/invite/:token" element={<AcceptInvitePage />} />
+
                   {/* Public legal pages */}
                   <Route path="/impressum" element={<LegalPage />} />
                   <Route path="/datenschutz" element={<LegalPage />} />