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>

.gitignore

@@ -21,6 +21,9 @@ backend/.idea/
 *.swp
 *.swo
 
+# Local development
+backend/src/*/appsettings.Local.json
+
 # OS
 Thumbs.db
 .DS_Store