chore: expand PyPI/GitHub metadata for discoverability

[greyllmmoder]

Summary

• pyproject.toml: +10 keywords (iata-bcbp, drug-serialization, supply-chain, healthcare-barcode, python-barcode, aztec-barcode, gs1-barcode, qr-code-alternative, boarding-pass-barcode, barcode-api)
• pyproject.toml: +6 classifiers (Healthcare Industry, Manufacturing, Console, Printing, Office/Business, Scientific/Engineering)
• pyproject.toml: added Documentation and Changelog sidebar URLs for PyPI
• README: decode-fast added to install section
• README: BCBP builder section added to API Reference
• README: changelog updated to Latest [1.2.0]
• GitHub: description updated with keyword-rich text
• GitHub: topics expanded to full 20 (added python3, pdf, iata-bcbp, gs1-barcode, healthcare, supply-chain, drug-serialization)

Notes

No code changes — metadata and docs only. Requires new PyPI publish to make keywords/classifiers/URLs live.