feat: sign webhooks for API channel and agentbots (#13892)

Account webhooks sign outgoing payloads with HMAC-SHA256, but agent bot
and API inbox webhooks were delivered unsigned. This PR adds the same
signing to both.

Each model gets a dedicated `secret` column rather than reusing the
agent bot's `access_token` (for API auth back into Chatwoot) or the API
inbox's `hmac_token` (for inbound contact identity verification). These
serve different trust boundaries and shouldn't be coupled — rotating a
signing secret shouldn't invalidate API access or contact verification.

The existing `Webhooks::Trigger` already signs when a secret is present,
so the backend change is just passing `secret:` through to the jobs.
Shared token logic is extracted into a `WebhookSecretable` concern
included by `Webhook`, `AgentBot`, and `Channel::Api`. The frontend
reuses the existing `AccessToken` component for secret display. Secrets
are admin-only and excluded from enterprise audit logs.

### How to test

Point an agent bot or API inbox webhook URL at a request inspector. Send
a message and verify `X-Chatwoot-Signature` and `X-Chatwoot-Timestamp`
headers are present. Reset the secret from settings and confirm
subsequent deliveries use the new value.

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/controllers/api/v1/accounts/agent_bots_controller.rb

@@ -34,6 +34,10 @@ class Api::V1::Accounts::AgentBotsController < Api::V1::Accounts::BaseController
     @agent_bot.reload
   end
 
+  def reset_secret
+    @agent_bot.reset_secret!
+  end
+
   private
 
   def agent_bot

app/controllers/api/v1/accounts/inboxes_controller.rb

@@ -66,6 +66,12 @@ class Api::V1::Accounts::InboxesController < Api::V1::Accounts::BaseController
     head :ok
   end
 
+  def reset_secret
+    return head :not_found unless @inbox.api?
+
+    @inbox.channel.reset_secret!
+  end
+
   def destroy
     ::DeleteObjectJob.perform_later(@inbox, Current.user, request.ip) if @inbox.present?
     render status: :ok, json: { message: I18n.t('messages.inbox_deletetion_response') }