feat(server): add configurable data retention policies with scheduled cleanup

[martsokha]

Context

The server performs soft deletes but does not enforce configurable retention policies with scheduled cleanup. The runtime's compliance requirements call for automatic deletion at expiry.

Requirements

• Per-workspace configurable retention periods for:
  • Original (pre-redaction) content
  • Redacted output (independent schedule from originals)
  • Audit records (longer retention, e.g., 7 years HIPAA, 6 years SOX)
• Zero-retention mode: originals discarded immediately after processing completes
• Scheduled background task that identifies expired resources and permanently removes them
• Audit records are never deleted before their configured retention period expires
• Retention policy changes are themselves recorded in the audit trail

References

• Server docs: ARCHITECTURE.md - Data Retention
• Runtime: COMPLIANCE.md Section 5