chore(exchange): persist an audit-trail record for manual/direct-API RI exchange executes (AWS + Azure)

[cristim]

Context

Neither the AWS executeExchange handler (internal/api/handler_ri_exchange.go) nor the new Azure executeAzureExchange handler (PR LeanerCloud/cloud-commitments-cli#1515, closes LeanerCloud/cloud-commitments-cli#596) writes a ledger/audit row when an exchange is executed directly via the API (as opposed to the approve/reject scheduled-exchange flow, which does persist via RIExchangeRecord / ExecuteApprovedExchange).

This is a parity gap for both providers, not something LeanerCloud/cloud-commitments-cli#596 introduced -- the AWS direct-execute path already has the same gap.

Scope

• Decide whether direct-API executes should write to the same ri_exchange_history (or equivalent) table the scheduled-approval flow uses, or a separate audit log.
• Persist: subscription/account, source reservation IDs, target SKU/region/term, session ID, net payable + currency, status, executed-by (session/API key), timestamp.
• Apply to both the AWS and Azure execute handlers for consistency.

Notes

• Not a blocker for feat(azure/ri-exchange): find compatible offerings and execute exchange (refs #473) cloud-commitments-cli#596 (execution itself works and is guardrailed; this is about after-the-fact operator visibility / compliance trail).
• See h.config.GetRIExchangeHistory / RIExchangeRecord in internal/config for the existing schema the scheduled-approval flow writes to.