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

.env.example

@@ -0,0 +1,26 @@
+# Docker Compose configuration for Helix.
+# Copy to `.env` and adjust:  cp .env.example .env
+
+# PostgreSQL
+PG_DATABASE=helix
+PG_USER=helix
+PG_PASSWORD=helix
+
+# Redis (keep empty for a local no-password Redis)
+REDIS_PASSWORD=
+
+# JWT signing secret — change this in any non-local deployment
+TOKEN_SECRET=please-change-this-secret
+
+# Microsoft Entra ID single sign-on (optional for local runs)
+MSAL_CLIENT_ID=
+MSAL_TENANT_ID=
+
+# S3-compatible object storage (optional — requires enabling `app.asset.enabled`
+# in docker/application-prod.yml and providing the credentials below)
+S3_ACCESS_KEY_ID=
+S3_SECRET_ACCESS_KEY=
+
+# Host ports
+SERVER_PORT=8080
+WEB_PORT=80