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>

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "dashboard/unifi",
     "description": "UniFi network management, WiFi stats, and captive portal authentication for the Dashboard platform",
-    "version": "1.5.3",
+    "version": "1.5.4",
     "type": "library",
     "license": "MIT",
     "autoload": {