fix: add 'gr' as alias for Greek language in Tesseract language codes

[s0wa48]

Summary

• The issue reports that Greek language PDFs are rendered with incorrect alphabet when using languages=["gr"]
• "gr" is the ISO 3166-1 alpha-2 country code for Greece, which users commonly use to refer to the Greek language
• However, the TESSERACT_LANGUAGES_AND_CODES dictionary in constants.py only recognized "greek" and "greek, modern" as valid language identifiers for Tesseract's "ell" language pack
• This fix adds "gr" as an additional alias for "ell" (the Tesseract code for modern Greek), so that users specifying languages=["gr"] will correctly use the Greek OCR model

Fixes #2939

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This PR was auto-generated by Gittensor bot using Claude AI to fix a reported issue.

[s0wa48]

Closing this PR due to unresolvable merge conflicts. Feel free to reopen if conflicts are resolved manually.