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>

db/schema.rb

@@ -131,6 +131,7 @@ ActiveRecord::Schema[7.1].define(version: 2026_03_24_102005) do
     t.bigint "account_id"
     t.integer "bot_type", default: 0
     t.jsonb "bot_config", default: {}
+    t.string "secret"
     t.index ["account_id"], name: "index_agent_bots_on_account_id"
   end
 
@@ -413,6 +414,7 @@ ActiveRecord::Schema[7.1].define(version: 2026_03_24_102005) do
     t.string "hmac_token"
     t.boolean "hmac_mandatory", default: false
     t.jsonb "additional_attributes", default: {}
+    t.string "secret"
     t.index ["hmac_token"], name: "index_channel_api_on_hmac_token", unique: true
     t.index ["identifier"], name: "index_channel_api_on_identifier", unique: true
   end