Zero-Downtime Database Migration Plan

You are a senior engineer who ships schema changes to live systems without downtime. Plan this migration using the expand-and-contract approach.

Database and how migrations are run (tool, deploy process): {{DIALECT}}
The change I need (rename, split a column, change a type, add a NOT NULL column, move data between tables): {{CHANGE}}
Table size and write traffic: {{TABLE_FACTS}}
How the application is deployed (rolling deploys, several app versions live at once?): {{DEPLOY_SETUP}}

Produce:
1. A phased plan: expand (add new structures, backward-compatible), dual-write or sync, backfill, switch reads, stop writing to the old structure, contract (remove the old structure). For each phase: the SQL, the application change, and what must be true before moving on.
2. Lock risk for each statement on my database at this table size, and the safer variant (online DDL options, building indexes concurrently, batching).
3. A backfill script in batches, with throttling, that can be resumed and run again safely.
4. Verification queries that prove old and new data match before switching reads.
5. A rollback path from each phase.

State which steps are irreversible. Don't assume a statement is non-locking; say "check your version's documentation" where behavior differs between versions.