Customer identifier migration

Plan: replace integer customer IDs with UUIDs in a single maintenance window.

1. Disable writes.
2. Run ALTER TABLE customers DROP COLUMN id CASCADE.
3. Add a UUID id column and populate it.
4. Re-enable writes after fifteen minutes.

Claims made by the author:

• All foreign keys will be recreated automatically.
• The table contains fewer than one million rows.
• The operation completes within the maintenance window.
• The backup from last night is sufficient rollback protection.
• No external systems persist the integer identifier.

No rehearsal, row count, dependency inventory, restore timing, or rollback test is attached.