feat: add cleanup command to remove expired db entries

[bjoern-m]

Description

This PR adds a new cleanup command to the backend, enabling automated database cleanup for expired entities.

Implementation

• New cleanup command:
  • Implemented using cobra CLI framework.
  • Supports dry-run mode and execution mode (--run).
  • Allows specifying individual tables to clean up (--tables flag).
• Added cleanup logic for:
  • audit_logs: Uses a retention period from the configuration.
  • flows and webauthn_session_data: Deletes expired entries based on timestamps.
• Configuration updates:
  • Introduced audit_log.retention in config.yaml (default: 720 hours).
  • Added validation for retention duration in config_audit_log.go.
• Persistence layer updates:
  • Extracted FlowPersister from flowdb.go and moved it to flow_persister.go.
  • Updated AuditLogPersister to support batch deletion of expired logs.

Additionally when creating a new audit log entry, the created_at and updated_at timestamps are now in UTC.

Tests

1. Run hanko cleanup --tables=audit_logs --run to perform a real cleanup.
2. Run hanko cleanup without --run to simulate cleanup.
3. Modify config.yaml to test different retention settings.