#124 Fix: Event deduplication at storage level

LONG-TERM FIX for guaranteed event uniqueness. Problem: Even with idempotency keys, race conditions can cause duplicate events. Changes needed: 1. Add unique constraint on (workflow_run_id, step_name, event_type, attempt) 2. Create event deduplication trigger function 3. Add event_hash column for content-based deduplication 4. Implement background job to clean up duplicate events 5. Update queries to handle potential duplicates gracefully Parent: #116