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

server/Dockerfile

@@ -0,0 +1,28 @@
+FROM amazoncorretto:21 AS build
+
+ARG ARTEFACT_VERSION='1.0.0'
+WORKDIR /home/app
+
+COPY gradlew .
+COPY gradle gradle
+COPY gradle.properties .
+COPY build.gradle.kts .
+COPY settings.gradle.kts .
+COPY gradle/libs.versions.toml gradle/libs.versions.toml
+
+RUN chmod +x gradlew && ./gradlew dependencies --no-daemon
+
+COPY src src
+RUN ./gradlew bootJar --no-daemon -PartefactVersion=${ARTEFACT_VERSION}
+
+FROM amazoncorretto:21-alpine
+
+WORKDIR /app
+
+COPY --from=build /home/app/build/libs/helix-server-${ARTEFACT_VERSION}.jar app.jar
+
+EXPOSE 8080
+
+ENV JAVA_OPTS="-XX:+UseContainerSupport -XX:MaxRAMPercentage=75.0"
+
+ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS -jar app.jar"]