fix: paginated list limit hard to change

[SychO9]

Fixes #3861

Changes proposed in this pull request:

• This PR creates a single source of truth for the limit (page size) of API lists.
• The source of truth/limit value is always from the backend controller limit. That way, changing the limit there affects the rest, even frontend listings.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.