feat: add data freshness check

[flitzpiepe93]

🚀 New Feature: Freshness Check

This PR introduces a new aggregate-level data quality check that validates whether a dataset has been updated recently enough, based on a configurable time threshold.

✅ What’s Included

• FreshnessCheck implementation (aggregate check)
• Flexible configuration of time threshold via:
  • interval: positive integer
  • period: one of "year", "month", "week", "day", "hour", "minute", "second"
• Declarative and programmatic config support via FreshnessCheckConfig
• Unit tests covering:
  • Valid & outdated timestamps
  • Empty / null-only columns
  • Literal enforcement and config validation
• Sphinx documentation with YAML and Python examples

🔍 Example Usage

FreshnessCheckConfig(
    check_id="updated-recently",
    column="last_updated",
    interval=2,
    period="day"
)

💡 Why This Matters

This check is inspired by the dbt freshness concept and helps detect:

• Stale or delayed data ingestion
• Broken or lagging ETL jobs
• Unexpected pipeline pauses

It ensures that datasets meet minimum freshness requirements, allowing pipelines to fail fast if data is outdated.

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Let me know if you'd like me to squash the commits or adjust the scope! ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

📢 Thoughts on this report? Let us know!

[tongqqiu]

LGTM