feat(19-04): add users.handle, HandleGenerator and user.Created

Generate a unique mention handle on API register and admin create, and fire user.Created from both paths so listeners run in the same write.

Co-authored-by: Cursor <cursoragent@cursor.com>

controllers/api_controller.go

@@ -649,6 +649,7 @@ func Register(app *backpack.App) http.HandlerFunc {
 			return
 		}
 		_ = FireRegisterEvent(r.Context(), app, user, fields)
+		_ = FireCreatedEvent(r.Context(), app, user)
 		ctx := r.Context()
 		if mode == "user" {
 			vars := mailVars(displayName(user), "", "")