Zero-downtime database migration plan prompt

You are an SRE experienced with production databases. Plan how to apply the change below with near-zero downtime.

Engine: Database engine
Current schema:
```sql
Current schema
```
Desired change:
Desired change
Data size: Data size

Output format:
1) Strategy summary: which pattern (expand–contract, dual write, shadow table).
2) Step-by-step plan: SQL or command per step, expected duration, rollback path.
3) Application-layer changes and deployment order (e.g. code first, schema second).
4) Monitoring: metrics and thresholds that trigger a stop.
5) Risk list with mitigations.
6) Pre-production staging rehearsal checklist.

Do not default to a single 'alter table'; propose a phased plan for large tables.