Support "Notification on Post" for following accounts

[malkoG]

Summary

Add support for "Notification on Post" functionality, similar to Mastodon and Misskey's feature where users can opt-in to receive notifications whenever a specific account publishes new content.

Motivation

• Users who want to use Hackerspub as a newsletter subscription management service need a way to get notified when followed accounts publish new content.
• Users who want to closely follow people of interest should be able to opt-in to notifications for specific accounts, rather than relying on manually checking their timeline.
• This is a well-established pattern in the fediverse (Mastodon's "bell" icon on profiles, Misskey's similar feature) and users coming from those platforms will expect it.

Scope

This feature should cover notifications for:

• Notes (short-form posts)
• Articles (long-form posts)

Context

• Relevant area: Notification system, follow/subscription management, ActivityPub integration
• Prior art: Mastodon's "Notify me when this person posts" bell toggle on profile pages, Misskey's equivalent feature

[malkoG]

Implementation consideration: Inbound ActivityPub activities

The notification trigger should also account for inbound Create(Note), Create(Article), and Create(Question) (Poll) activities received from remote actors. When a user has "Notify on Post" enabled for a remote account, the server should check the incoming Create activity and generate a notification accordingly — not just for local posts.