chore: restructure into monorepo and add open-source documentation

- Rename helix-server -> server and helix-web -> web
- Remove helix-api (bruno collection and docs site)
- Merge per-module .gitignore files into the root .gitignore
- Add README, AGENTS, CONTRIBUTING and MIT LICENCE
- Add full-stack docker-compose (postgres, redis, server, web via Caddy)
  with web Dockerfile, Caddyfile and Compose server config
- Keep the Gradle wrapper jar tracked after the .gitignore merge
- Copy gradle.properties into the server Docker build image

web/Caddyfile

@@ -0,0 +1,13 @@
+:80 {
+	# API reverse proxy — strip the /api prefix and forward to the server container
+	handle_path /api/* {
+		reverse_proxy server:8080
+	}
+
+	# SPA static files with client-side routing fallback
+	handle {
+		root * /srv
+		try_files {path} /index.html
+		file_server
+	}
+}