Add backend submodule

Use backend submodule from organization.

.gitmodules

@@ -1,6 +1,9 @@
 [submodule "packages/app-web"]
 	path = packages/app-web
 	url = git@code.modal.sh:zeichen/app-web.git
+[submodule "packages/backend"]
+	path = packages/backend
+	url = git@code.modal.sh:zeichen/backend.git
 [submodule "packages/core"]
 	path = packages/core
 	url = git@code.modal.sh:zeichen/core.git

packages/backend

@@ -0,0 +1 @@
+Subproject commit b8a37cd92621a8768a32d7ee8859883158fa5f31