fix(purchase): skip duplicate purchase_history insert on re-drive and add a unique index #704

Summary

Re-drive and root retry insert duplicate purchase_history rows. ExistingCommitment is never read, and there is no unique index to stop the second insert. Spend, savings and YTD totals are double counted.

Where

At commit b50ca38:

• internal/purchase/execution.go:686-699
• store_postgres.go:2023-2061

Failure scenario

An execution is re-driven (or a root is retried) after the commitment already exists. The code inserts a second history row for the same purchase instead of skipping it, so dashboards and savings reports count it twice. Found by source reading; not run against Postgres.

Suggested fix

• Skip the history insert when ExistingCommitment is set.
• Add a dedupe migration and a unique index on (provider, account_id, purchase_id).
• Refuse a root retry when completed children exist.

Severity

High: financial totals are wrong, and duplicate rows corrupt data that other features read.

Related: #267 (duplicate purchase_id rows wedging the marketplace listing slot). That issue covers the marketplace-slot symptom; this one covers the insert path that creates the duplicates and the double counted spend.

Reviewer ID: MON-06 (money H6)
Found by the 2026-10-08 review fleet.