Add the nightly audit-record purge job

Python · Python · intermediate · greenfield

Adds the retention policy module and the nightly purge job that enforces it. The policy computes the oldest kept day from RETENTION_DAYS; the purge job selects everything older and deletes it. Ran it against a staging snapshot and the record counts dropped as expected.

An audit-logging service under a 90-day compliance retention rule. The purge job runs nightly; deletions are irreversible (no soft delete, no backup restore inside the audit store).

Requirements

• Compliance requires keeping audit records for the trailing 90 calendar days counting today; app/retention/policy.py is the single source of truth for what may be deleted, and the purge job must follow its contract exactly.
• select_purgeable(records, today) returns exactly the records the policy allows deleting — deleting a record the policy says to keep is a compliance violation, the worse failure direction.
• records is a list of dicts with 'id' (str) and 'day' (datetime.date, the record's calendar day); it may be empty.
• purge_expired(store, today) deletes the purgeable records via store.delete(id) and returns how many it deleted. store.all_records() returns the full record list.

Files touched

• app/retention/policy.py
• app/retention/purge.py

--- app/retention/policy.py
+"""Data-retention policy for audit records.
+
+Compliance retains audit records for the trailing RETENTION_DAYS calendar
+days, COUNTING TODAY: with RETENTION_DAYS = 90, records dated today or any
+of the 89 days before it are inside the window and must be kept.
+"""
+
+from datetime import date, timedelta
+
+RETENTION_DAYS = 90
+
+
+def oldest_kept_day(today: date) -> date:
+    """Return the oldest day that must still be KEPT (inclusive).
+
+    Records dated on this day or later are inside the retention window and
+    must never be deleted. Only records dated STRICTLY BEFORE this day are
+    eligible for purging.
+    """
+    return today - timedelta(days=RETENTION_DAYS - 1)
+
--- app/retention/purge.py
+"""Nightly purge job: delete audit records that fell out of retention."""
+
+from datetime import date
+
+from app.retention.policy import oldest_kept_day
+
+

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