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/Dockerfile

@@ -0,0 +1,29 @@
+# Build the React SPA, then serve it with Caddy (static files + /api reverse proxy).
+FROM node:22-alpine AS build
+
+WORKDIR /app
+
+ARG VITE_API_BASE_URL=/api
+ARG VITE_MSAL_CLIENT_ID=
+ARG VITE_MSAL_TENANT_ID=
+ARG VITE_APP_TITLE=Helix
+ARG VITE_DEFAULT_REGION_ABBREVIATION=GB
+
+ENV VITE_API_BASE_URL=$VITE_API_BASE_URL \
+    VITE_MSAL_CLIENT_ID=$VITE_MSAL_CLIENT_ID \
+    VITE_MSAL_TENANT_ID=$VITE_MSAL_TENANT_ID \
+    VITE_APP_TITLE=$VITE_APP_TITLE \
+    VITE_DEFAULT_REGION_ABBREVIATION=$VITE_DEFAULT_REGION_ABBREVIATION
+
+COPY package.json pnpm-lock.yaml ./
+RUN corepack enable && pnpm install --frozen-lockfile
+
+COPY . .
+RUN pnpm build
+
+FROM caddy:2-alpine
+
+COPY --from=build /app/dist /srv
+COPY Caddyfile /etc/caddy/Caddyfile
+
+EXPOSE 80