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-tunnel.ps1

@@ -0,0 +1,4 @@
+Write-Host "SSH Tunnel: localhost:5433 -> server:5432 (dev_db)" -ForegroundColor Cyan
+Write-Host "Press Ctrl+C to stop"
+Write-Host ""
+ssh -N -L 5433:127.0.0.1:5432 -o ServerAliveInterval=60 deploy@31.131.18.254