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>

deploy/docker-compose.local.yml

@@ -0,0 +1,9 @@
+services:
+  gb-local-backend:
+    build: ../backend
+    container_name: gb-local-backend
+    ports:
+      - "127.0.0.1:5000:5000"
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Development
+      - ConnectionStrings__Default=Host=host.docker.internal;Port=5433;Database=dev_db;Username=dev_user;Password=dev_pass_vB6nM3qP8yW2rT9k