feat: add recalculate_representative_docs() method with configurable count [stacks on #2514]

[pidefrem]

What does this PR do?

feat: add recalculate_representative_docs() method with configurable count

_save_representative_docs hardcodes nr_samples=500 and nr_repr_docs=3. Users cannot control how many representative documents are stored per topic.

This is frequently requested: #552, #848, #1767. In #1767, @MaartenGr indicated he prefers a standalone recalculation function (not a constructor parameter). PR #2130 attempted a constructor-only approach but stalled.

Changes:

1. New public method: recalculate_representative_docs(nr_repr_docs=3, nr_samples=500) — recalculates and replaces representative_docs_ after training. Users can call it with different counts without re-fitting.
2. Constructor defaults nr_repr_docs (default 3) and nr_repr_docs_nr_samples (default 500) — used during fit_transform().

Backward compatible: defaults reproduce the current behavior exactly.

Fixes #2500

Note: Pairs well with #2496 (dedup fix for _extract_representative_docs) but works standalone.

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
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• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?

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⚠️ Draft — stacks on #2514. This branch includes the MMR batching commit from #2514 (which itself carries the #2496 fix) as prerequisites (marked [prereq]), so the diff here shows that work too. Once #2514 merges, I'll rebase onto master and the prereq commits drop out, leaving only the recalculate_representative_docs() change. Marking ready for review after #2514 lands.