feat(yami): add dataDir option for storing stack state on separate disk

- Bind-mounts /var/lib/{gitea,postgresql} under configurable dataDir
- gitea-runner intentionally excluded: its DynamicUser + StateDirectory
  conflicts with pre-existing mount points ("File exists")
- Runner token moved to /var/lib/gitea-runner-secret/ to avoid the same
  symlink/StateDirectory conflict inside /var/lib/gitea-runner
- Enable the stack on this host with dataDir = /mnt/ssd_110/yami

configuration.nix

@@ -7,7 +7,7 @@
     ./desktop.nix
     ./nvidia.nix
     ./shell.nix
-    # ./yami-dev-stack.nix  # uncomment to enable Yami dev stack, then set services.yami-dev-stack below
+    ./yami-dev-stack.nix 
   ];
 
   nix = {
@@ -98,5 +98,11 @@
   # Docker is now managed by yami-dev-stack.nix when that module is enabled.
   # services.yami-dev-stack = { enable = true; user = "chexit"; };
 
+  services.yami-dev-stack = {                               
+    enable = true;                                                                                        
+    user   = "chexit";
+    dataDir = "/mnt/ssd_110/yami";
+  };  
+
   system.stateVersion = "25.11";
 }

yami-dev-stack.nix

@@ -13,9 +13,20 @@ in
       type = types.str;
       description = "User that will own runner token and be added to docker group";
     };
+
+    dataDir = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      example = "/mnt/ssd_110/yami";
+      description = ''
+        If set, bind-mounts /var/lib/{gitea,postgresql,gitea-runner} under this
+        path so all stack data lives on a separate disk. Directories are created
+        automatically; existing data must be rsync'd manually before enabling.
+      '';
+    };
   };
 
-  config = mkIf cfg.enable {
+  config = mkIf cfg.enable (mkMerge [ {
 
     # ── Systemd target ─────────────────────────────────────────────────────────
     # Manual-only: nothing in wantedBy, so it never starts on boot.
@@ -128,7 +139,11 @@ in
       enable    = true;
       name      = "home-runner";
       url       = "http://localhost:3000";
-      tokenFile = "/var/lib/gitea-runner/token";
+      # NOT under /var/lib/gitea-runner — that path is systemd-managed for the
+      # service (DynamicUser + StateDirectory means it becomes a symlink to
+      # /var/lib/private/gitea-runner). Pre-creating it ourselves breaks the
+      # service with "Failed to set up special execution directory: File exists".
+      tokenFile = "/var/lib/gitea-runner-secret/token";
       labels    = [
         "ubuntu-latest:docker://node:20"
         "native:host"
@@ -143,12 +158,13 @@ in
     };
 
     # Runner uses DynamicUser, so there's no static gitea-runner user/group.
-    # tokenFile is loaded via systemd EnvironmentFile, so the file MUST be in
-    # env-file format, not a raw token:
+    # Token lives in a sibling dir (not /var/lib/gitea-runner — that's systemd's
+    # StateDirectory and pre-creating it breaks DynamicUser bind setup).
+    # The file MUST be in env-file format, not a raw token:
     #     TOKEN=<paste-registration-token-from-gitea-ui>
     # Get the token from Site Administration → Actions → Runners → Create new Runner.
     systemd.tmpfiles.rules = [
-      "d /var/lib/gitea-runner 0700 root root -"
+      "d /var/lib/gitea-runner-secret 0700 root root -"
     ];
 
     # ── Shell aliases ──────────────────────────────────────────────────────────
@@ -162,5 +178,31 @@ in
     # ── Firewall ───────────────────────────────────────────────────────────────
     # Merged with existing rules (Steam etc.); does not overwrite them.
     networking.firewall.allowedTCPPorts = [ 3000 2222 ];
-  };
+  }
+
+  # ── Optional: move all stack data to a separate disk via bind-mounts ─────────
+  (mkIf (cfg.dataDir != null) (
+    let
+      # gitea-runner intentionally NOT bind-mounted: its service uses
+      # DynamicUser + StateDirectory, which refuses to adopt an existing
+      # mount point ("Failed to set up special execution directory: File exists").
+      # Its data is tiny (token + .runner state) so it stays on the system disk.
+      subs = [ "gitea" "postgresql" ];
+      mkBind = name: {
+        name = "/var/lib/${name}";
+        value = {
+          device  = "${cfg.dataDir}/${name}";
+          fsType  = "none";
+          options = [ "bind" ];
+        };
+      };
+    in {
+      # Ensure target dirs exist on the data disk before mounts happen.
+      systemd.tmpfiles.rules = map (n: "d ${cfg.dataDir}/${n} 0755 root root -") subs
+        ++ [ "d ${cfg.dataDir} 0755 root root -" ];
+
+      fileSystems = listToAttrs (map mkBind subs);
+    }
+  ))
+  ]);
 }