Chat middleware demo

The web host is running. Use an API client or the PowerShell example in README.md to test the chat endpoints.

The default backend simulates bot replies and agent handoff. Conversations and media are kept in memory and disappear when the application restarts.

RequestPurpose
POST /api/chat/openStart a conversation
POST /api/chat/sendSend text or media (multipart form)
GET /api/chat/messagesPoll for updates
POST /api/chat/closeClose a conversation
GET /api/chat/media/{id}Download media

All API requests require the X-Api-Key header configured in Web.config. Send agent to try the simulated handoff.