Add webhook support for confirmation email

[wozniak-slawomir]

Hi everyone, hi @knadh. I decided to initiate creating this very basic feature, let me know what you think. I tried to follow contributing guidelines.

Add outgoing webhook for subscription confirmation

This PR adds support for outgoing webhooks that fire when a subscriber
confirms their subscription (both single and double opt-in).

Features

• New internal/webhooks package for HTTP webhook delivery
• Configurable URL, timeout, and retry settings
• Fires on subscription confirmation (not on initial signup for double opt-in)
• Settings UI in admin panel under Settings → Webhooks
• Database migration for existing installations

Payload Example

{
  "event": "subscription.confirmed",
  "timestamp": "2024-12-23T00:00:00Z",
  "data": {
    "subscriber": { "id": 1, "email": "...", ... },
    "lists": [{ "id": 1, "name": "Newsletter" }]
  }
}

What needs to be done still?

• Translations
• Tests?
• other stuff

Please let me know.