Add a module

Each direct subpackage of {{BASE_PACKAGE}} is a module. ./mvnw verify fails if another module reaches into its nested packages or if modules form a cycle.

1. Pick a name: a noun for the capability, such as billing, not billingservice or common. Check that no existing module owns it; the Modules section of AGENTS.md lists them.
2. Create the layout:

   {{BASE_PACKAGE}}.<module>            public API: the service other modules call, the records it takes and returns, its events
   {{BASE_PACKAGE}}.<module>.internal   everything else: implementation, entities, repositories, controllers

   Keep the top-level package small. Entities and repositories never go in it.
3. Let other modules talk to it in one of two ways only: call a type in its top-level package, or listen for an event it publishes (@ApplicationModuleListener). Prefer events when the caller doesn't need an answer.
4. Add src/test/java/{{BASE_PATH}}/<module>/<Module>ModuleTests.java, annotated @ApplicationModuleTest, which starts only this module. Test the module through its public API.
5. Add one line for the module to the Modules section of AGENTS.md.
6. Run ./mvnw verify.