Create Access controller admin endpoints

[magush27]

Add admin-protected endpoints for managing Access clients through Postman or future admin panel.

File: api/controllers/access.js

Endpoints:

1. POST /api/admin/access-clients - Create new Access client
   • Requires admin authentication
   • Creates AccessClient and updates boards with accessCode
   • Validates rootBoardId exists
2. GET /api/admin/access-clients - List all clients with stats
   • Returns all clients with board counts, expiry status
   • Populates rootBoard and createdBy
3. PUT /api/admin/access-clients/:code - Update client
   • Updates allowed fields: isActive, isListedInApp, subscription dates, branding
4. GET /api/admin/access-clients/:code/stats - Get detailed statistics
   • Returns client info, access counts, board list with tile counts

Tasks:

• Implement createAccessClient with board association
• Implement listAccessClients with computed fields
• Implement updateAccessClient
• Implement getAccessClientStats