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

packages/arrtrix/pkg/connector/config_test.go

@@ -1,23 +0,0 @@
-package connector
-
-import "testing"
-
-func TestConfigDefaultsApplyRadarrWebhookPath(t *testing.T) {
-	var cfg Config
-
-	cfg.applyDefaults()
-
-	if cfg.Webhooks.Radarr.Path == "" {
-		t.Fatal("expected radarr webhook path default to be set")
-	}
-}
-
-func TestConfigValidateRejectsEnabledWebhookWithoutSecret(t *testing.T) {
-	cfg := Config{}
-	cfg.Webhooks.Radarr.Enabled = true
-	cfg.applyDefaults()
-
-	if err := cfg.Validate(); err == nil {
-		t.Fatal("expected missing secret to fail validation")
-	}
-}