feat: add CleanReport JSON export

[WilliamK112]

Summary

• add first-class CleanReport.to_json(**kwargs) and CleanReport.write_json(path, **kwargs) helpers without changing the existing to_dict() payload
• cover JSON-string options and a UTF-8 file round-trip using a real cleaning report
• update the batch CSV example, quickstart, and Unreleased changelog to use the official API

Closes #21.

Validation

• pytest tests/test_report.py --no-cov — 12 passed
• ruff check . — passed
• mypy src/freshdata/report.py — passed (with NumPy 2.4; the latest 2.5 stubs require a newer mypy target than this repository's configured Python 3.10 target)
• python examples/08_csv_automation.py — completed and wrote 3 JSON audit logs
• git diff --check — passed
• full required fast lane with .[dev,ml]: 4,054 passed, 6 skipped, 12 deselected; 93.16% aggregate coverage (93% required)

The new serialization lines are exercised by the focused tests; src/freshdata/report.py measured 96% coverage in the full run.

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[JohnnyWilson16]

Hey @WilliamK112,

Thanks for another great PR!

Really appreciate how thorough you were here—not just adding to_json and write_json, but also handling non-primitive serialization defaults (default=str for timestamps in metadata), updating the quickstart and example script, and adding solid round-trip tests. It makes exporting clean audit logs so much smoother.

Everything looks super clean and ready to go. Merging this in! 🚀