Removed go.sum from the COPY cache layer — go mod download -x creates it
when absent. Added empty backend/go.sum for local tooling (gopls, go build).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds Compose: Build & Up, Up, Down, Logs tasks for both Windows and Linux,
allowing the complete app to be tested locally via docker-compose on port 8080.
Updates README task table to reflect all available tasks.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>