Webhook notifications to Slack not work

[stssp29]

🐛 Bug Report

Describe the bug
Webhook notifications to Slack return 400 Bad Request when using a Slack Incoming WebHook URL.
The “Test” button in the webhook settings reports success in logs, but no message is posted to the Slack channel. When a real event occurs (e.g. creating a card), the webhook delivery fails with HTTP 400 from Slack.

To Reproduce
Steps to reproduce the behavior:

1. Go to Settings → Webhooks in Kan.bn
2. Create a new webhook with URL set to Incoming WebHook URL (https://hooks.slack.com/services/...)
3. Click the Test button in the webhook configuration but nothing appears in the configured Slack channel but log show successful
   {"level":30,"time":1774504669319,"pid":1,"hostname":"3429a4a61482","module":"trpc","procedure":"webhook.test","type":"mutation","duration":44,"userId":"b8d9e3e1-2be5-436a-8b62-79e7c1255719","msg":"tRPC OK"}
4. Create a new card on a board and move card around nothing appears in the configured Slack channe but see error in logs
   {"level":50,"time":1774504787134,"pid":1,"hostname":"3429a4a61482","module":"webhook","url":"https://hooks.slack.com/services/...","event":"card.moved","error":"400 Bad Request","statusCode":400,"msg":"Webhook delivery failed"}

Expected behavior

1. Clicking Test should send Test message to Slack and a message should appear in the target channel.
2. Real events (e.g. card.created) should be delivered successfully to Slack in the target channel.

Environment (please complete the following information):

• OS: Linux Fedora 43
• Browser Firefox 148.0.2, Microsoft Edge 146.0.3856.72
• App version v0.5.6

Additional context
I test my webhook with curl and it work fine, I'm not sure look like payload is not set or invalid but I can't see option to custom set the payload