Document redirect service architecture

[Copilot]

Adds architecture documentation for the redirect service.

• Request flow
  • Documents cookie, geolocation, and default-group resolution.
  • Describes redirect status precedence and URI preservation.
• Core components
  • Covers HTTP handlers, group management, load balancing, and health checks.
• Operations
  • Defines configuration, endpoints, CLI options, build usage, and repository layout.