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/application/use-cases/sign-up.use-case.ts

@@ -1,5 +1,7 @@
 import { z } from "zod";
 
+import type { IEventBus } from "@repo/core-events";
+import { userSignedUpEvent } from "../../events/user-signed-up.event";
 import { AuthenticationError } from "../../entities/errors/auth";
 import { cookieSchema } from "../../entities/models/cookie";
 import { sessionSchema } from "../../entities/models/session";
@@ -34,6 +36,7 @@ export const signUpUseCase =
   (
     usersRepository: IUsersRepository,
     authenticationService: IAuthenticationService,
+    bus: IEventBus,
   ) =>
   async (input: SignUpInput): Promise<SignUpOutput> => {
     const existingUser = await usersRepository.getUserByUsername(input.username);
@@ -52,5 +55,13 @@ export const signUpUseCase =
 
     const { cookie, session } = await authenticationService.createSession(newUser);
 
+    // Auth is username-based — synthesize a deterministic email so the event
+    // payload validates against userSignedUpEventSchema.email().
+    await bus.publish(userSignedUpEvent, {
+      userId: newUser.id,
+      email: `${newUser.username}@example.local`,
+      signedUpAt: new Date().toISOString(),
+    });
+
     return signUpOutputSchema.parse({ session, cookie });
   };