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>

config/routes.rb

@@ -88,6 +88,7 @@ Rails.application.routes.draw do
           resources :agent_bots, only: [:index, :create, :show, :update, :destroy] do
             delete :avatar, on: :member
             post :reset_access_token, on: :member
+            post :reset_secret, on: :member
           end
           resources :contact_inboxes, only: [] do
             collection do
@@ -221,6 +222,7 @@ Rails.application.routes.draw do
             post :sync_templates, on: :member
             get :health, on: :member
             post :register_webhook, on: :member
+            post :reset_secret, on: :member
             if ChatwootApp.enterprise?
               resource :conference, only: %i[create destroy], controller: 'conference' do
                 get :token, on: :member