Not all webhook providers use X-Hub-Signature-256 or X-Webhook-Signature.
Add an optional signature_header field so users can specify which header
carries the HMAC signature. When set, that exact header is checked;
when unset, the existing auto-detect fallback is preserved. The custom
header is also excluded from stored/exposed payload headers.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Webhook ingest endpoint was completely unauthenticated. Add an optional
`secret` field to BoardWebhook. When configured, inbound requests must
include a valid HMAC-SHA256 signature in X-Hub-Signature-256 or
X-Webhook-Signature headers. Uses hmac.compare_digest for timing safety.
Includes migration to add the secret column.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
