Implement r5500 media stack configuration with NFS and Docker services

- Added configuration for media stack on r5500, including paths for Jellyfin, Sonarr, Radarr, and other media services.
- Integrated NFS client for accessing Jellyfin libraries from nixdesk.
- Established Docker services for Dispatcharr and Organizr, including necessary user and group setups.
- Created systemd services for managing media directories and ensuring proper permissions.

modules/hosts/r5500/_private/media-disk.nix

@@ -0,0 +1,52 @@
+# Media stack storage on r5500: btrfs subvolume @media-stack on the OS disk (sda4).
+{ config, pkgs, lib, ... }:
+let
+  btrfsUuid = "934a5ec3-4bab-49c3-96c9-c857c50076ba";
+  btrfsDevice = "/dev/disk/by-uuid/${btrfsUuid}";
+  # Created under subvol=@ → full path is @/@media-stack (not a top-level @media-stack).
+  mediaSubvol = "@/@media-stack";
+in
+{
+  # Create @media-stack before /mnt/media-stack is mounted.
+  systemd.services.r5500-media-stack-subvolume = {
+    description = "Create btrfs subvolume @media-stack on sda4 if missing";
+    before = [ "mnt-media\\x2dmedia\\x2dstack.mount" ];
+    wantedBy = [ "local-fs-pre.target" ];
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true;
+    };
+    script = ''
+      rootMount="/mnt/.r5500-btrfs-root"
+      mkdir -p "$rootMount"
+      if ! mountpoint -q "$rootMount"; then
+        mount -o subvol=@ "${btrfsDevice}" "$rootMount"
+        umountAfter=1
+      else
+        umountAfter=0
+      fi
+      if ! ${pkgs.btrfs-progs}/bin/btrfs subvolume show "$rootMount/@media-stack" >/dev/null 2>&1; then
+        ${pkgs.btrfs-progs}/bin/btrfs subvolume create "$rootMount/@media-stack"
+      fi
+      if [ "$umountAfter" -eq 1 ]; then
+        umount "$rootMount"
+      fi
+    '';
+  };
+
+  # Optional 1 TiB cap (run once after first boot if desired):
+  #   sudo btrfs quota enable /mnt/media-stack && sudo btrfs qgroup limit 1T /mnt/media-stack
+
+  fileSystems."/mnt/media-stack" = {
+    device = btrfsDevice;
+    fsType = "btrfs";
+    neededForBoot = false;
+    options = [
+      "subvol=${mediaSubvol}"
+      "compress=zstd"
+      "noatime"
+      "nofail"
+      "x-systemd.device-timeout=30"
+    ];
+  };
+}