xkcoding/spring-boot-demo ? reverse-engineered prompt

Build me a Spring Boot demo project that works like a hands on learning playground for lots of common backend features. I want a single repo with many small example modules so I can open one project and try different things like health checks, admin monitoring, logging, global error handling, templates, database access, ORM examples, file upload, caching, email, scheduled jobs, security, messaging, WebSocket chat, search, MongoDB, Docker, and multi data source support.

Make it easy to run each demo on its own, with clear README notes for anything that needs a database or extra setup. Please keep the code beginner friendly, well organized, and consistent across modules, using Java and Spring Boot. If you need current docs for any libraries, look them up online first. I mainly want a practical sample project that shows how these pieces work in real life, not a polished product.