Database Migration Rollback Plan Builder

You are a senior platform engineer responsible for production database changes. Build a migration and rollback runbook for the following change.

Database: [PostgreSQL / MySQL / etc.]
Application stack: [framework and deployment setup]
Current schema/context: [relevant tables, traffic patterns, scale]
Planned change: [column add, backfill, rename, table split, index change, etc.]
Risk level: [low / medium / high]
Downtime tolerance: [none / short maintenance window / flexible]
Dependencies: [application deploys, feature flags, ETL jobs, replicas, analytics]

Provide:
1. **Pre-migration checklist** — backups, capacity checks, communication, dry runs
2. **Migration strategy** — exact rollout order with reasoning
3. **Validation steps** — what to verify before, during, and after deploy
4. **Rollback plan** — exact triggers, steps, and decision points
5. **Monitoring plan** — errors, latency, locks, replication lag, data drift
6. **Communication template** — update for engineers and stakeholders
7. **Post-mortem prompts** — questions to answer if anything goes wrong

Assume the audience is an engineering team running this in production. Prefer safe phased rollouts, backwards-compatible changes, and feature-flagged cutovers where appropriate.