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

.editorconfig

@@ -0,0 +1,6 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8