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/javascript/dashboard/store/modules/agentBots.js

@@ -183,6 +183,17 @@ export const actions = {
       return null;
     }
   },
+
+  resetSecret: async ({ commit }, botId) => {
+    try {
+      const response = await AgentBotsAPI.resetSecret(botId);
+      commit(types.EDIT_AGENT_BOT, response.data);
+      return response.data;
+    } catch (error) {
+      throwErrorMessage(error);
+      return null;
+    }
+  },
 };
 
 export const mutations = {

app/javascript/dashboard/store/modules/inboxes.js

@@ -366,6 +366,16 @@ export const actions = {
     );
     return response.data;
   },
+  resetSecret: async ({ commit }, inboxId) => {
+    try {
+      const response = await InboxesAPI.resetSecret(inboxId);
+      commit(types.default.EDIT_INBOXES, response.data);
+      return response.data;
+    } catch (error) {
+      throwErrorMessage(error);
+      return null;
+    }
+  },
 };
 
 export const mutations = {