refactor: split server modules into smaller more manageable files

modules/nixos-modules/server/immich/impermanence.nix

@@ -0,0 +1,25 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  mediaLocation = "/var/lib/immich";
+in {
+  config = lib.mkIf (config.services.immich.enable && config.host.impermanence.enable) {
+    assertions = [
+      {
+        assertion = config.services.immich.mediaLocation == mediaLocation;
+        message = "immich media location does not match persistence";
+      }
+    ];
+    environment.persistence."/persist/system/root" = {
+      directories = [
+        {
+          directory = mediaLocation;
+          user = "immich";
+          group = "immich";
+        }
+      ];
+    };
+  };
+}