fix(youko): wait for data mounts before tmpfiles setup (#498)

systemd-tmpfiles-setup only orders itself after local-fs.target, but the nofail /data submounts can complete later. It then sees /data as the read-only fallback tmpfs and fails to create the Transmission, Immich, RustFS and restic directories at boot.

Add RequiresMountsFor for the data mounts and cover the generated unit dependency with an eval test.

outputs/x86_64-linux/tests/youko-tmpfiles/expected.nix

@@ -0,0 +1,10 @@
+{ ... }:
+{
+  requiresMountsFor = [
+    "/data/apps"
+    "/data/apps-snapshots"
+    "/data/backups"
+    "/data/fileshare"
+    "/data/fileshare/public"
+  ];
+}

outputs/x86_64-linux/tests/youko-tmpfiles/expr.nix

@@ -0,0 +1,11 @@
+{
+  lib,
+  outputs,
+  ...
+}:
+let
+  unit = outputs.nixosConfigurations.youko.config.systemd.services.systemd-tmpfiles-setup;
+in
+{
+  requiresMountsFor = builtins.sort builtins.lessThan unit.unitConfig.RequiresMountsFor;
+}