#122 Fix: Add idempotency keys to event emission

SHORT-TERM FIX for event duplication during retries. Problem: EventLogger emits new events with incrementing sequence numbers on each task retry. This causes 'event stabilization' checks to fail during disruption testing. Changes needed: 1. Add idempotency_key column to absurd_event_log table 2. Generate deterministic idempotency key: hash(workflow_run_id, step_name, event_type) 3. Use ON CONFLICT DO NOTHING for duplicate events 4. Update EventLogger.log_event() to use idempotency keys Parent: #116