[Security] Implement HIPAA-Compliant Audit Logging #14

Problem

Audit logs don't track who performed actions (missing user/service identity).

Impact

• Cannot comply with healthcare audit requirements
• No accountability
• Regulatory violations

Solution

Track identity in all audit logs:

• User/service making request
• Timestamp with timezone
• Action taken
• Data affected
• Result (success/failure)

Acceptance Criteria

• Identity added to all events
• Audit trail queryable by user
• Timestamps ISO 8601 with timezone
• HIPAA compliance checklist passed