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>

backend/src/GBSite.Api/Program.cs

@@ -1,4 +1,5 @@
 var builder = WebApplication.CreateBuilder(args);
+builder.Configuration.AddJsonFile("appsettings.Local.json", optional: true, reloadOnChange: true);
 
 builder.Services.AddControllers();
 builder.Services.AddOpenApi();

backend/src/GBSite.Api/Properties/launchSettings.json

@@ -5,7 +5,7 @@
       "commandName": "Project",
       "dotnetRunMessages": true,
       "launchBrowser": false,
-      "applicationUrl": "http://localhost:5224",
+      "applicationUrl": "http://localhost:5000",
       "environmentVariables": {
         "ASPNETCORE_ENVIRONMENT": "Development"
       }