feat: reorganize admin settings with tabbed interface and documentation

- Reorganized admin settings into tabbed interface (General, Security, Payment Methods)
- Vertical tabs on desktop, horizontal scrollable on mobile
- Moved Payment Methods from separate menu to Settings tab
- Fixed admin profile reuse and dashboard blocking
- Fixed maintenance mode guard to use AppConfig model
- Added admin auto-redirect after login (admins → /admin, users → /)
- Reorganized documentation into docs/ folder structure
- Created comprehensive README and documentation index
- Added PWA and Web Push notifications to to-do list

apps/api/src/health/health.controller.ts

@@ -1,7 +1,9 @@
 import { Controller, Get } from '@nestjs/common';
 import { PrismaService } from '../prisma/prisma.service';
+import { SkipMaintenance } from '../common/decorators/skip-maintenance.decorator';
 
 @Controller('health')
+@SkipMaintenance()
 export class HealthController {
   constructor(private readonly prisma: PrismaService) {}