Registers the freshly-minted webhook URL with the external provider on the operator's behalf

Closes the gap where the operator otherwise has to paste the URL into BotFather / Slack app settings / Twilio console manually. Providers with a programmatic registration API (Telegram `setWebhook` today, more later) get a single-click install; providers without one (Slack Events API, Teams Bot Framework) return 400 and the FE renders manual instructions instead. The FE supplies the URL it just received from `:generateWebhookUrl` / `/webhook:rotate` via the request body — Fruxon doesn't store the plaintext key, so the round-trip through the operator's browser is the only way the BE can pass the full URL to the provider. Returns 200 with `applied=false` + `errorMessage` when the provider rejects (auth failure, malformed URL, rate limit) — the FE surfaces the message inline and offers the manual-install fallback.

curl -X POST "https://api.fruxon.com/v1/tenants/string/integrations/string/configs/497f6eca-6276-4993-bfeb-53cbbbba6f08:applyWebhookToProvider" \  -H "Content-Type: application/json" \  -d '{}'

{
  "applied": false,
  "providerResponse": "string",
  "errorMessage": "string"
}

{
  "type": "string",
  "title": "string",
  "status": 0,
  "detail": "string",
  "instance": "string",
  "property1": null,
  "property2": null
}

{
  "type": "string",
  "title": "string",
  "status": 0,
  "detail": "string",
  "instance": "string",
  "property1": null,
  "property2": null
}