Restructure daemon into cmd/, internal/api, internal/config

[enegalan]

Summary

Reorganize the Go backend from a single main.go into a proper daemon layout.

Tasks

• Move entry point to backend/cmd/calf/main.go
• Add backend/internal/api for HTTP handlers and routing
• Add backend/internal/config for configuration loading
• Keep go run ./cmd/calf (or equivalent) working from backend/
• Update DEVELOPMENT.md if paths or commands change

Acceptance criteria

• Daemon starts with the new structure
• No business logic left in main.go beyond wiring and startup
• Package layout follows standard Go project conventions

Parent

Part of #1