fix(history): purchase_history rows stamped convertible while the class setting was standard stay unsellable #524

Summary

Before merged #509, savePurchaseHistory hard-coded convertible into purchase_history.offering_class for the legacy ec2 slug. Rows written while the global offering class was standard say convertible but are Standard, so they cannot be sold on the marketplace.

Location (origin/main 496d9d7)

• Stamp: internal/purchase/execution.go:961 (now the configured class).
• Guard: internal/api/handler_marketplace.go:113 isKnownNonStandardOfferingClass (defined :492) returns 400, and the UI hides Sell.
• Lazy re-stamp for empty class: populateOfferingClass (internal/api/handler_marketplace.go:308).

Failure scenario

Every AWS ec2 row written while global_config.offering_class='standard' (from the class setting's introduction, commit 425bc2f, 2026-07-17, until #509) is stored as convertible but is Standard. The user cannot sell a Standard RI they own. Global config history is not stored, so the true class can only come from AWS.

Options (owner decision)

a. UPDATE purchase_history SET offering_class = NULL WHERE provider='aws' AND service IN ('ec2','compute') AND offering_class='convertible', so populateOfferingClass re-fetches and re-stamps lazily. Cost: genuinely Convertible rows show a Sell button that 400s once.
b. One-off job calling FetchOfferingClass per row.

Also

On an idempotent re-drive the stamp records the class configured at retry time, not the class bought. Needs an "already existed" signal on the go library's PurchaseResult; the linked go issue will be filed separately.

Context

Related: #112 (closed, same defect before #509).

Acceptance

No AWS ec2 purchase_history row carries an offering_class that disagrees with AWS, verified by a query or job report.