docs: Comprehensive documentation update for Fly.io deployment

[StuartF303]

Updates all deployment documentation to reflect the complete single-container architecture and all fixes implemented:

README.md:

• Add detailed Fly.io deployment architecture section
• Document health check endpoint (/health)
• Add SignalR WebSocket URLs for local and cloud
• Expand troubleshooting with SignalR and health check issues
• Document automatic API URL configuration

DEPLOYMENT.md:

• Add detailed request routing section
• Expand health check failure troubleshooting
• Document SignalR WebSocket connection debugging
• Add frontend API URL issue resolution
• Include supervisorctl commands for process management

docs/deployment/fly-io-deployment.md:

• Add comprehensive "Key Implementation Details" section
  • Health check endpoint implementation
  • Dynamic API URL configuration
  • WebSocket support for SignalR with timeouts
  • Environment-aware CORS configuration
• Massively expand troubleshooting section:
  • Health check failures with diagnosis steps
  • SignalR WebSocket debugging procedures
  • Frontend localhost issue resolution
  • Build failures and deployment issues
  • Logs and monitoring commands
  • Emergency rollback procedures
  • Common error messages reference

docs/deployment/ARCHITECTURE.md (NEW):

• Complete deployment architecture documentation
• Detailed container architecture diagram
• Request flow diagrams for static files, API, and WebSocket
• Component configuration details (Fly.io, nginx, supervisor, Docker)
• Frontend and backend configuration with code examples
• GitHub Actions deployment pipeline documentation
• Networking and security details
• Scaling and performance considerations
• Monitoring and observability guide
• Disaster recovery procedures
• Cost analysis
• Troubleshooting reference matrix

All documentation now reflects:

• Health check endpoint (/health) in API
• Dynamic API URL using HostEnvironment.BaseAddress
• WebSocket support with 7-day timeouts
• Environment-aware CORS settings
• Single-container deployment architecture