Add backend/README.md with Quick Start Guide

[tgrunnagle]

Summary

Created a comprehensive README for the backend Go game server that provides quick start instructions, development workflow, and API endpoint reference. The README focuses on practical usage rather than design details, referencing the existing architecture documentation for deeper technical information.

Changes

• Added backend/README.md with:
  • Project overview and description
  • Prerequisites and installation steps (Go 1.24+, Task, golangci-lint)
  • Quick start guide for running the server
  • Testing instructions (unit tests, short mode)
  • Development workflow using Task commands (be:check, be:lint, be:format, be:vet)
  • Complete API endpoint reference table covering WebSocket, game management, training, and bot action endpoints
  • Reference to docs/architecture/backend.md for architectural details

Test Plan

• Verify all Task commands referenced in README are valid
• Confirm installation steps are accurate
• Check that API endpoint table matches actual implementation
• Ensure architecture document link is correct

Closes: #107