nixos/stalwart-mail: Add dataDir option

I want to move it elsewhere.

nixos/modules/services/mail/stalwart-mail.nix

@@ -8,7 +8,6 @@ let
   cfg = config.services.stalwart-mail;
   configFormat = pkgs.formats.toml { };
   configFile = configFormat.generate "stalwart-mail.toml" cfg.settings;
-  dataDir = "/var/lib/stalwart-mail";
   useLegacyStorage = lib.versionOlder config.system.stateVersion "24.11";
 
   parsePorts =
@@ -46,6 +45,14 @@ in
         By default, the module is configured to store everything locally.
       '';
     };
+
+    dataDir = lib.mkOption {
+      type = lib.types.path;
+      default = "/var/lib/stalwart-mail";
+      description = ''
+        Data directory for stalwart
+      '';
+    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -63,15 +70,15 @@ in
           {
             # structured data in SQLite, blobs on filesystem
             db.type = lib.mkDefault "sqlite";
-            db.path = lib.mkDefault "${dataDir}/data/index.sqlite3";
+            db.path = lib.mkDefault "${cfg.dataDir}/data/index.sqlite3";
             fs.type = lib.mkDefault "fs";
-            fs.path = lib.mkDefault "${dataDir}/data/blobs";
+            fs.path = lib.mkDefault "${cfg.dataDir}/data/blobs";
           }
         else
           {
             # everything in RocksDB
             db.type = lib.mkDefault "rocksdb";
-            db.path = lib.mkDefault "${dataDir}/db";
+            db.path = lib.mkDefault "${cfg.dataDir}/db";
             db.compression = lib.mkDefault "lz4";
           };
       storage.data = lib.mkDefault "db";
@@ -124,11 +131,11 @@ in
         preStart =
           if useLegacyStorage then
             ''
-              mkdir -p ${dataDir}/data/blobs
+              mkdir -p ${cfg.dataDir}/data/blobs
             ''
           else
             ''
-              mkdir -p ${dataDir}/db
+              mkdir -p ${cfg.dataDir}/db
             '';
 
         serviceConfig = {