Make the user that triggered the bulk automatic translation the author

### Describe the problem

When using the bulk automatic translation, all actions (suggestions or actual translations) come from a technical `mt:` user.

This makes it impossible to have traceability who actually initiated and is responsible for these translation.

### Solution brainstorm

- Make the user who initiated the bulk automatic translation the author of any suggestions.
- Make the user also the author in commit messages.
- Add the provider used in the commit message, possibly with the [`Generated-by:`](https://www.apache.org/legal/generative-tooling.html#include-in-contributions) or `Co-authored-by:` tags.
- Track the machinery used for the translation in a separate metadata field and show it on suggestions.
- Get rid of the "technical" `mt:` users.

### Describe alternatives you have considered

- Add the user to a new metadata field and keep the author as-is: would also be possible, but makes authorship searches more difficult. The user should also be added with a [`Signed-off-by:`](https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---signoff) or some other form in the commit.
- Only log the triggering of the bulk automatic translation with the user to the component's history: makes it cumbersome to find out who did it by having to first scroll through a possibly very long list of automatic translations in the history first.

### Screenshots

Suggestion from a automatic translation with DeepL:


Commit message from a automatic translation with DeepL:
```
Author: DeepL
Date: Wed Jun 25 11:20:47 2025 +0000

Translated using Weblate (Portuguese)

Currently translated at 100.0% (33 of 33 strings)

Translation: My Project/My Component
Translate-URL: https://example.com/projects/my-project/my-component/pt/
```

### Additional context

_No response_

Start with the bulk automatic translation flow that creates suggestions and commits, then inspect how the initiating user and the mt: technical user are selected. Decide on one authorship and metadata design from the listed alternatives, preserving provider attribution. Done means suggestions and commit history identify the initiator while recording the translation machinery.

Written by the indexing model from the issue text.