feat(auth): signUp publishes userSignedUpEvent

signUpUseCase now takes an IEventBus and publishes userSignedUpEvent
after creating the user (synthesizing email from username since auth
is username-based). Use case mocks-default in module.ts get a fresh
InMemoryEventBus per resolution; bind-production / bind-dev-seed wire
the shared bus passed by bindAll. Tests updated to inject
RecordingEventBus, including a new test that asserts publish on
success and silence on failure.

packages/auth/src/di/module.ts

@@ -1,4 +1,5 @@
 import { ContainerModule, type interfaces } from "inversify";
+import { InMemoryEventBus } from "@repo/core-events";
 
 import type { IUsersRepository } from "../application/repositories/users.repository.interface";
 import type { IAuthenticationService } from "../application/services/authentication.service.interface";
@@ -44,9 +45,13 @@ export const AuthModule = new ContainerModule((bind: interfaces.Bind) => {
   );
 
   bind<ISignUpUseCase>(AUTH_SYMBOLS.ISignUpUseCase).toDynamicValue((ctx) =>
+    // Default fallback uses a fresh InMemoryEventBus — real cross-feature
+    // wiring runs through bindProductionAuth / bindDevSeedAuth where the
+    // bindAll() dispatcher passes a shared bus instance.
     signUpUseCase(
       ctx.container.get<IUsersRepository>(AUTH_SYMBOLS.IUsersRepository),
       ctx.container.get<IAuthenticationService>(AUTH_SYMBOLS.IAuthenticationService),
+      new InMemoryEventBus(),
     ),
   );