database.WithTx: rollback if func returns error

Previously, this only rolled back if the called function panicked. This
updates it to also roll back if the called function returns an error.

webhooks/legacy_test: verify headers are recorded

webhooks/legacy: address Postgres-specific issues

webhooks/legacy_test.go: style

webhooks/legacy: update only affected delivery

Forgot to add this where clause, whoops!

Add legacy webhooks worker implementation