Align built-in pretranslation with API path

[baha-bouali]

Fixes #4526

What changed

Following the points agreed in the issue, the built-in pretranslation now follows the
same rules as the pretranslation upload API.

Behavior

• Strings with unreviewed suggestions are pretranslated. A suggestion matching the
  pretranslation becomes the pretranslation and keeps its author; other suggestions are
  kept, only deactivated.

• Fuzzy translations and older, different pretranslations are rejected and replaced; a
  matching one is converted instead.

• Strings with an approved translation are skipped, before calling machine translation.

• A pretranslation that still fails checks after the retry with preserved placeables is
  not stored.

• Strings whose pretranslation was rejected are still never pretranslated again, and
  Translate Toolkit checks stay disabled.

  Structure

• decide_pretranslation() (in sync/upload.py) holds the decision both paths share.

• reject_translations() is a shared helper for rejecting and logging.

• The built-in path has its own write, write_pretranslations(), since it differs from
  the API: bot authors, per-resource stats and latest translation, no check results to
  store, and dropping strings instead of raising.

• Writes happen in order: reject → deactivate → update → create, so a string never has
  two active translations.

  Choices

Those still need your confirmation:

• Rejections are recorded as the Sync system user. Insights ignores it, so these
  automatic rejections don't count against pretranslation quality.
• The built-in write also takes lock_import_target(), so it waits for an upload of the
  same strings instead of colliding with it.
• bulk_run_checks() is gone: everything stored has passed all checks, and the stale
  warnings and errors of converted suggestions are deleted.
• The latest translation of each resource is the newest one written, since a converted
  suggestion can be months old.

Docs

With these changes, two docs no longer described the behavior correctly, so I
updated docs/dev/api.md and docs/admin/managing-pretranslation.md to match.

Tests

This is a draft, and new tests will follow.

[baha-bouali]

A small note aside

This one took me roughly three weeks. Most of that time went into reading the code and studying the possible solutions. That was intentional: I wanted a deeper understanding of how pretranslation, uploads and the translation data fit together, and I got it.

[mathjazz]

Thanks @baha-bouali — the structure matches what we agreed, and the converted match correctly keeps its original author.

I'll wait for you to address the outcomes of our discussion before digging deeper.