feat(admin/webhooks): add/edit/delete UI with template editor and presets

New /admin/webhooks page lets a super-admin manage webhook subscriptions
without touching tinyice.json by hand. Replaces the legacy add/delete
form (which is still wired for backward compatibility).

- Sidebar entry + AdminLayout route under /admin/webhooks.
- List view shows enabled/disabled state, method, subscribed events,
  and per-row Test/Enable/Edit/Delete actions.
- Modal editor with: optional Name, URL, HTTP method, Content-Type,
  per-event subscription checkboxes, repeating header rows, large
  body-template textarea, and a click-to-insert placeholder helper
  driven by /api/webhooks/meta (so the available variables stay in
  sync with what the server actually emits per event).
- Preset dropdown applies a verified template (Discord, Slack, MS
  Teams, Telegram, ntfy, Pushover, TuneIn AIR, etc.) without
  overwriting a URL the operator has already typed.
- Test button POSTs to /api/webhooks/test to fire a sample payload
  for the webhook's first subscribed event — useful for verifying
  the template before relying on a real track change to trigger it.

Author: DatanoiseTV