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

docker-compose.yaml

@@ -0,0 +1,81 @@
+name: helix
+
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_DB: ${PG_DATABASE:-helix}
+      POSTGRES_USER: ${PG_USER:-helix}
+      POSTGRES_PASSWORD: ${PG_PASSWORD:-helix}
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${PG_USER:-helix} -d ${PG_DATABASE:-helix}"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+    restart: unless-stopped
+
+  redis:
+    image: redis:7-alpine
+    command: ["redis-server", "--appendonly", "yes"]
+    volumes:
+      - redis-data:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+    restart: unless-stopped
+
+  server:
+    build:
+      context: ./server
+      args:
+        ARTEFACT_VERSION: ${ARTEFACT_VERSION:-0.0.1}
+    environment:
+      SPRING_PROFILES_ACTIVE: prod
+      PG_HOST: postgres
+      PG_PORT: "5432"
+      PG_DATABASE: ${PG_DATABASE:-helix}
+      PG_USER: ${PG_USER:-helix}
+      PG_PASSWORD: ${PG_PASSWORD:-helix}
+      REDIS_HOST: redis
+      REDIS_PORT: "6379"
+      REDIS_PASSWORD: ${REDIS_PASSWORD:-}
+      TOKEN_SECRET: ${TOKEN_SECRET:-please-change-this-secret}
+      MSAL_CLIENT_ID: ${MSAL_CLIENT_ID:-}
+      MSAL_TENANT_ID: ${MSAL_TENANT_ID:-}
+      S3_ACCESS_KEY_ID: ${S3_ACCESS_KEY_ID:-}
+      S3_SECRET_ACCESS_KEY: ${S3_SECRET_ACCESS_KEY:-}
+    volumes:
+      # Mounts the Compose-specific server configuration (Spring Boot picks up
+      # config/application-{profile}.yml next to the working directory).
+      - ./docker/application-prod.yml:/app/config/application-prod.yml:ro
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+    ports:
+      - "${SERVER_PORT:-8080}:8080"
+    restart: unless-stopped
+
+  web:
+    build:
+      context: ./web
+      args:
+        VITE_API_BASE_URL: ${VITE_API_BASE_URL:-/api}
+        VITE_MSAL_CLIENT_ID: ${VITE_MSAL_CLIENT_ID:-}
+        VITE_MSAL_TENANT_ID: ${VITE_MSAL_TENANT_ID:-}
+        VITE_APP_TITLE: ${VITE_APP_TITLE:-Helix}
+        VITE_DEFAULT_REGION_ABBREVIATION: ${VITE_DEFAULT_REGION_ABBREVIATION:-GB}
+    depends_on:
+      - server
+    ports:
+      - "${WEB_PORT:-80}:80"
+    restart: unless-stopped
+
+volumes:
+  postgres-data:
+  redis-data: