feat: async CSV user import

[niStee]

Closes #1138

Problem

Bulk adding users via the textarea form caps out quickly due to PHP max execution timeouts.

Solution

This PR introduces a robust CSV importer that chunks the upload from the frontend.

• Uploads the CSV file once to a temporary server path.
• JS polls the backend in batches.
• PHP reads chunks using fseek() and fgetcsv().
• Safely supports mapping existing users via a collision behavior dropdown (Skip, Merge Groups, Replace Groups).
• Can optionally send welcome emails.

[CatoTH]

I can review / test this later. What I saw: it's english-only. It would be nice to make this translatable; pattern would be something like:
PHP Side:
messages/en/admin.php - define the keys and translations. Relevant quirk right now: it needs to be marked as js=true to be exposed into JS.
No need to define other translations, though feel free to add Dutch if you want :)

JS SIde:

import translations from "/js/vue/Translate.vue.js";
...
translations.getTranslation("user", "pw_min_x_chars");

Also, it would be good to have a sample CSV somewhere in docs/ .

[niStee]

I've added the translations using Translate.vue.js and added a sample CSV to the docs/ directory! I also merged the latest main branch so it's completely up to date and ready for review.

[CatoTH]

There were still some issues - but I could fix them in this Commit: cf4efb0 . I can either merge my MR, or you can cherry pick it if you want and then I can merge yours.

If you want to add yourself to the AUTHORS file, go ahead. :)