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/views/api/v1/models/_inbox.json.jbuilder

@@ -113,6 +113,7 @@ end
 ## API Channel Attributes
 if resource.api?
   json.hmac_token resource.channel.try(:hmac_token) if Current.account_user&.administrator?
+  json.secret resource.channel.try(:secret) if Current.account_user&.administrator?
   json.webhook_url resource.channel.try(:webhook_url)
   json.inbox_identifier resource.channel.try(:identifier)
   json.additional_attributes resource.channel.try(:additional_attributes)