Add local development environment with switchable backend

Set up local dev workflow: SSH tunnel to dev DB, Next.js API proxy
via rewrites, Docker or native backend on port 5000. Update and
restructure project documentation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/local-backend-dotnet.ps1

@@ -0,0 +1,6 @@
+Write-Host "Starting backend (dotnet run) on http://localhost:5000" -ForegroundColor Cyan
+Write-Host "Requires: SSH tunnel running (scripts/local-tunnel.ps1)"
+Write-Host ""
+Push-Location "$PSScriptRoot\..\backend"
+dotnet run --project src/GBSite.Api
+Pop-Location