Add support for Postmark bounce notifications via webhooks

[tosie]

Please excuse me from not posting a feature issue first. The implementation was not that much work and I felt it would be easier to just write code and talk about that.

This PR adds support for Postmark's bounce notifications.

Currently, Listmonk has support for SES and Sendgrid bounce notifications. I use Postmark and would like to use Listmonk, so it made sense to contribute something to this project that might be beneficial for other users, too.

There are two main areas of this PR:

1. Adding a setting to enable/disable the Postmark webhook service (including translations in the frontend).
2. Implementing a Postmark webhook service to handle bounce notifications.

There is also a migration (v2.6.0) that adds the setting key to the DB.

[knadh]

Thank you for the PR, @tosie. I'll review this in the next couple of weeks for the upcoming release.

[knadh]

PR looks good @tosie. One minor correction. The upcoming version is v2.5.0. Please remove the v2.6.0 migration and add it to v2.5.0 instead.

[tosie]

PR looks good @tosie. One minor correction. The upcoming version is v2.5.0. Please remove the v2.6.0 migration and add it to v2.5.0 instead.

Done ✅

[knadh]

Thanks! Tested this. Was reviewing the Postmark docs before merging and realised that it supports HTTP BasicAuth for webhooks. Without this, the current implementation would be open to abuse (unauthenticated POST).

[tosie]

You are right. I would try to add that, but might need help with the translations …

[knadh]

Hi @tosie. I've been experimenting with GPT-4 auto-translating missing strings in language packs using English as the base. The results are pretty good. Will finish this script and bundle in the repo soon. You can add just the English strings and leave it.

[tosie]

This PR is getting big, @knadh :-). Basic auth for Postmark webhooks is implemented now. You can take another look, if you'd like to.

[tosie]

Rebased on current master.

[knadh]

I'd made a few changes to this PR last week. Only got time to finish it just now. Could you please take a look? 0289c01

[tosie]

Your commit removes the HTTP basic auth handler completely. I'll try to re-add that to the bounce package (inside of the postmark handler code).

[knadh]

Ah, my bad. Added it here: a96b243

[tosie]

Looks good 👍. Thanks!

[tosie]

Closed in favor of #1485.