Implement F# backend using Giraffe + Fable.Remoting with proper separation: Validation → Domain (pure logic) → Persistence (I/O) → API.
Use when implementing server-side logic, API endpoints, or business rules.
Ensures validation at boundaries, pure domain functions, and proper error handling with Result types.
Creates code in src/Server/ files: Validation.fs, Domain.fs, Persistence.fs, Api.fs.