feat: add auto-grow-root service for live disk resize

Adds systemd service that automatically grows the root filesystem on boot:
- Detects partition layout and grows partition if needed (growpart)
- Supports both ext4 (resize2fs) and btrfs filesystems
- Works with VIRTIO-SCSI disk (/dev/sda)
- Enables seamless live disk resize from the hosting platform

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

configuration.nix

@@ -89,6 +89,38 @@
   # Enable the OpenSSH daemon.
   services.openssh.enable = true;
 
+  # Auto-grow root filesystem on boot (for live disk resize support)
+  # Works with VIRTIO-SCSI (/dev/sda) - detects partition layout automatically
+  systemd.services.auto-grow-root = {
+    description = "Auto-grow root filesystem";
+    wantedBy = [ "multi-user.target" ];
+    after = [ "local-fs.target" ];
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true;
+    };
+    path = with pkgs; [ cloud-utils parted e2fsprogs btrfs-progs util-linux ];
+    script = ''
+      set -euo pipefail
+
+      ROOT_DEV=$(findmnt -n -o SOURCE /)
+      ROOT_DISK=$(lsblk -no PKNAME "$ROOT_DEV" | head -1)
+
+      # If partitioned, grow partition first
+      if [[ "$ROOT_DEV" =~ [0-9]$ ]]; then
+        PART_NUM=$(echo "$ROOT_DEV" | grep -oE '[0-9]+$')
+        growpart "/dev/$ROOT_DISK" "$PART_NUM" 2>/dev/null || true
+      fi
+
+      # Grow filesystem
+      FSTYPE=$(findmnt -n -o FSTYPE /)
+      case "$FSTYPE" in
+        ext4) resize2fs "$ROOT_DEV" 2>/dev/null || true ;;
+        btrfs) btrfs filesystem resize max / 2>/dev/null || true ;;
+      esac
+    '';
+  };
+
 
   # This value determines the NixOS release from which the default
   # settings for stateful data, like file locations and database versions