feat: implement comprehensive admin functionality

- Add admin role system with role and isAdmin fields to users table
- Create admin_actions audit log table for tracking all admin operations
- Implement admin CLI tool for user management (create, promote, demote, list, check)
- Add admin authentication with role-based access control
- Create admin service layer with system statistics and user management
- Implement user impersonation system with proper security checks
- Add admin API endpoints for user management and system statistics
- Create admin dashboard UI with overview, users, and action logs
- Fix admin stats endpoint and user deletion with proper foreign key handling
- Add admin link to navigation bar for admin users

Database:
- Add role and isAdmin columns to users table
- Create admin_actions table for audit trail
- Migration script: add-admin-functionality.js

CLI:
- src/backend/scripts/admin-cli.js - Admin user management tool

Backend:
- src/backend/services/admin.service.js - Admin business logic
- Updated auth.service.js with admin helper functions
- Enhanced index.js with admin routes and middleware
- Export sqlite connection from config for raw SQL operations

Frontend:
- src/frontend/src/routes/admin/+page.svelte - Admin dashboard
- Updated api.js with adminAPI functions
- Added Admin link to navigation bar

Security:
- Admin-only endpoints with role verification
- Audit logging for all admin actions
- Impersonation with 1-hour token expiration
- Foreign key constraint handling for user deletion
- Cannot delete self or other admins
- Last admin protection

src/backend/config.js

@@ -122,6 +122,8 @@ export const db = drizzle({
   schema,
 });
 
+export { sqlite };
+
 export async function closeDatabase() {
   sqlite.close();
 }