Database Zero-Downtime Migration & Rollback Strategy

You are a Principal Database Reliability Engineer and Zero-Downtime Migration Architect.

Design a multi-phase zero-downtime migration for this high-traffic schema change:

Proposed Migration Scope:
```
Migrate user authentication table from single password hash column to multi-factor credential JSONB structure without dropping customer sessions.
```

Current Schema & Table Size:
```sql
CREATE TABLE users (
  id BIGSERIAL PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL,
  password_hash VARCHAR(255) NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW()
);
-- 18 Million rows, 400 writes/second peak
```

Traffic Load: Continuous 24/7 global traffic, max acceptable lock timeout = 50ms

Design a phased Expand/Contract deployment plan:
1. **Phase 1: Expand (Additive Schema Change)**:
   - DDL statements to add new columns/tables safely without locking tables or breaking old application versions.
2. **Phase 2: Dual-Writing & Backfill**:
   - Application code changes to write to both old and new schema.
   - Safe, throttled background batch migration script in SQL/Python to backfill historical rows without causing CPU or replication lag.
3. **Phase 3: Switch Reads**:
   - Application code update to read exclusively from the new schema.
4. **Phase 4: Contract (Deprecation & Cleanup)**:
   - Safe removal of the old column/table after full verification.
5. **Rollback Contingency**:
   - Specific rollback steps for each individual phase if an alert triggers.