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/types/index.ts

@@ -144,6 +144,18 @@ export interface AdminUser {
   username: string;
   email: string;
   mustChangePassword?: boolean;
+  role?: {
+    name: string;
+    displayName: string;
+  };
+  roleId?: number;
+  roleName?: string;
+  roleDisplayName?: string;
+  isActive?: boolean;
+  lastLogin?: string | null;
+  lastLoginIp?: string | null;
+  createdAt?: string;
+  createdByUsername?: string;
 }
 
 export interface LoginResponse {
@@ -178,5 +190,32 @@ export interface ApiError {
   }>;
 }
 
+// Role and Permission types
+export interface AdminRole {
+  id: number;
+  name: string;
+  displayName: string;
+  description?: string;
+  isSystem?: boolean;
+  priority?: number;
+}
+
+export interface AdminPermissions {
+  role: {
+    name: string;
+    displayName: string;
+  } | null;
+  permissions: string[];
+}
+
+export interface AdminInvitation {
+  id: number;
+  email: string;
+  roleName: string;
+  invitedBy: string;
+  expiresAt: string;
+  createdAt: string;
+}
+
 // Export protection types
 export * from './protection';