Refactor arrtrix webhook to use fixed path and remove legacy config

- Switch arrtrix webhook to a fixed path: /_arrtrix/webhook
- Remove Radarr-specific and secret-based config from arrtrix
- Simplify connector and webhook handler logic
- Update NixOS module to drop legacy webhook config
- Add new tests for generic arrtrix webhook handler

modules/nixos/temp/services/arrtrix/default.nix

@@ -15,11 +15,6 @@
   settingsFormat = pkgs.formats.json {};
 
   defaultConfig = {
-    network.webhooks.radarr = {
-      enabled = false;
-      path = "/_arrtrix/webhooks/radarr";
-      secret = "";
-    };
     bridge = {
       command_prefix = "!arr";
       relay.enabled = true;