TSA-296: [BE] Update the lexicon when a page is verified

[arturobdev]

Closes #296

Summary

• Added lexicon entry repository and domain entities.
• Added entity extraction from transcription text and structured output.
• Updates lexicon entries on confirm and correct.
• Uses corrected transcription values when available.
• skip does not contribute to the lexicon.
• Lexicon updates run inside the same transaction as page verification.
• Populates lexiconAdded with the entries affected by the verification.
• Respects preset.settings.minDistinctPages.

Testing

• Confirming a page creates lexicon entries.
• Repeated words across different pages increment freq and distinct_pages.
• Entries become available in context after reaching minDistinctPages.
• Correcting a page uses the corrected text / structured output instead of the original values.
• Replaying the same correct action does not change lexicon counters.
• Skipping a page does not modify the lexicon.
• lexiconAdded contains the entries added or updated by the action.
• buildContext() includes learned lexicon entries in later-page context.

Lexicon entries after verifying multiple pages:

lexiconAdded returned by the verify endpoint:

Learned lexicon entries included in later-page context:

[RomanNabukhotniidev]

god job!

The base branch was changed.

[yurii-tymoshevskyi]

lgtm