Push notification subscriptions should support more granular / arbitrary policies

[trwnh]

Pitch

Possible policies that don't currently exist:

• Mutuals
• List members
• Some subset of followers (although this one needs a way to create such audiences as a pre-requisite)
• Arbitrary accounts (on a per-account basis)

On the most general end, have a way to create custom policies that check for membership within a set that contains accounts and/or contains other auto-updating sets (follows, lists, followers, audiences, mutuals).

An alternative that requires less changes: support only some additional policies, like list members or mutuals (although this is ultimately less flexible and leads to more complexity)

Motivation

Sometimes you don't want to get notifications from all your followers or all your follows.

Etc

This is partially tangent with #22937 but that issue seems to be focused on overriding conversation mutes

[Strgnoff]

I referred to this here issue in #7692 and I am crossposting my explanation here:

Both of these together are IMHO vital to improve visibility and help people establish more close-knitted networks with each other. Especially accounts that are smaller and post less frequent have difficulties to gain traction, because followers who use Mastodon only occasionally (like, once per day or even just once per week) will probably have almost all other notifications disabled (nobody wants notification spam on their phone or desktop; we're all busy with work), but would really like to receive instant notifications in case of new postings from select accounts (close friends, trusted partners on lists) or particular niche hashtags.

I personally find this kind of granular control over lists and notifications to be a key feature in making Mastodon more user-friendly and helping to make it accessible for users who are not hardcore social media power users, but who want to ditch closed source networks (like Instagram and Facebook) while still being able to get reliable, if occasional updates from their social circle (of those, who migrated to Mastodon as well, as everyone should).

These are core features which belong in the official web interface and official Android/iOS apps, as those are the interfaces that the vast majority of non-power-users are going to use.