fix(webhooks): add missing columns; add pre-save URL test endpoint

* The model+validation referenced tracked_clients and templates columns
  but they were never in the unifi_webhook_configs migration. Any save
  attempt that included those keys 500'd with "Unknown column".
  Added an additive migration (idempotent) that adds both as nullable
  json columns.
* New POST /settings/webhooks/test-url endpoint takes a url+secret in
  the body and fires the standard test payload. Lets operators validate
  their endpoint before saving the row — useful when first wiring up
  Google Chat, Slack, etc.

v1.5.4.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

database/migrations/2026_05_24_000002_add_templates_and_tracked_clients_to_unifi_webhook_configs.php

@@ -0,0 +1,32 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('unifi_webhook_configs', function (Blueprint $table) {
+            if (! Schema::hasColumn('unifi_webhook_configs', 'tracked_clients')) {
+                $table->json('tracked_clients')->nullable()->after('device_filter');
+            }
+            if (! Schema::hasColumn('unifi_webhook_configs', 'templates')) {
+                $table->json('templates')->nullable()->after('tracked_clients');
+            }
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('unifi_webhook_configs', function (Blueprint $table) {
+            if (Schema::hasColumn('unifi_webhook_configs', 'templates')) {
+                $table->dropColumn('templates');
+            }
+            if (Schema::hasColumn('unifi_webhook_configs', 'tracked_clients')) {
+                $table->dropColumn('tracked_clients');
+            }
+        });
+    }
+};