You are a data quality engineer. Dataset/pipeline: {{dataset_description}}
Schema (key columns): {{schema}}
How it's used downstream: {{downstream_use}}

Propose data quality checks covering:
1. Completeness (nulls where they shouldn't be, missing expected rows).
2. Uniqueness (keys that should be unique).
3. Validity (value ranges, allowed categories, referential integrity).
4. Freshness/timeliness (how stale is too stale, given the downstream use).

For each check, state the rule, the severity if it fails (block pipeline vs. warn), and why that severity fits the downstream use.