feat(core-shared): add retention-purge background job

Adds retention-purge.job.ts to packages/core-shared/src/payload/retention-purge/.
Walks every Payload collection's custom.retention.purgeSchedule, registers one
scheduled job per collection via IJobQueue, queries rows past their activeRetention
period (by createdAt for from-creation, updatedAt for from-last-access), and applies
pseudonymize (null PII fields) or hard-delete per postDeletion.action. Emits one
IAuditLog.record entry per processed row; gracefully skips when auditLog is absent.

Unit tests cover: schedule registration, trigger-type routing, hard-delete branch,
pseudonymize branch, audit emission, graceful auditLog skip, no-activeRetention
short-circuit, and input validation error cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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