feat: enhance webhook functionality for post events

[satputekuldip]

What kind of change does this PR introduce?

Feature – Webhooks are sent for all post lifecycle events (create, update, delete, publish, fail) to configured webhook URLs.

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Why was this change needed?

Users and automation tools (N8N, Make, Zapier, etc.) need to react when something happens to a post in Postiz. The app already has a Webhooks UI and says users can “get notified when something happens in Postiz via an HTTP request,” but post events were not consistently sent.

This change:

• Sends a standardized webhook for: post.created, post.updated, post.deleted, post.published, and post.failed.
• Uses a single payload shape: { event, timestamp, data } so integrations can rely on one contract.
• Keeps delivery fire-and-forget with error handling and logging so failed or slow endpoints don’t block create/update/delete/publish flows.

(Link the GitHub issue here if you created one, e.g. “Fixes #123” or “Related to #123.”)

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Other information:

• Implementation:
  • WebhooksService.sendPostEvent() loads org webhooks (filtered by integration), POSTs to each URL with the payload, and uses Promise.allSettled so one failure doesn’t block others.
  • PostsService triggers webhooks on create (post.created/updated) and delete (post.deleted).
  • Post workflow triggers webhooks on success (post.published) and on failure (post.failed), using the internal post id and proper error payload.
• Logging: Errors and non-2xx responses are logged; successful sends can be logged at debug level.
• Future: Optional retries or dead-letter handling could be added later if needed.

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Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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