jellyfin: restartTriggers on webhook plugin so install runs at activation

The jellyfin-webhook-install oneshot has 'wantedBy = jellyfin.service',
which only runs it when jellyfin (re)starts. On first rollout to a host
where jellyfin is already running, the unit gets added but never fires,
leaving the Webhook plugin files absent -- jellyfin-webhook-configure
then gets 404 from /Plugins/$GUID/Configuration and deploy-rs rolls back.

Pinning jellyfin.restartTriggers to the plugin package + install script
forces a restart whenever either derivation changes, which pulls install
in via the existing before/wantedBy chain.

services/jellyfin/jellyfin-qbittorrent-monitor.nix

@@ -13,7 +13,13 @@ in
 lib.mkIf config.services.jellyfin.enable {
   # Materialise the Jellyfin Webhook plugin into Jellyfin's plugins dir before
   # Jellyfin starts. Jellyfin rewrites meta.json at runtime, so a read-only
-  # nix-store symlink would EACCES — we copy instead.
+  # nix-store symlink would EACCES -- we copy instead.
+  #
+  # `wantedBy = [ "jellyfin.service" ]` alone is insufficient on initial rollout:
+  # if jellyfin is already running at activation time, systemd won't start the
+  # oneshot until the next jellyfin restart. `restartTriggers` on jellyfin pinned
+  # to the plugin package + install script forces that restart whenever either
+  # changes, which invokes this unit via the `before`/`wantedBy` chain.
   systemd.services.jellyfin-webhook-install = {
     before = [ "jellyfin.service" ];
     wantedBy = [ "jellyfin.service" ];
@@ -28,6 +34,13 @@ lib.mkIf config.services.jellyfin.enable {
     };
   };
 
+  systemd.services.jellyfin.restartTriggers = [
+    webhookPlugin.package
+    (webhookPlugin.mkInstallScript {
+      pluginsDir = "${config.services.jellyfin.dataDir}/plugins";
+    })
+  ];
+
   # After Jellyfin starts, POST the plugin configuration so the webhook
   # targets the monitor's receiver. Idempotent; runs on every boot.
   systemd.services.jellyfin-webhook-configure = {