feat(sandbox): full sandwich E2E test against real sandbox APIs (STA-214)

[Puneethkumarck]

Summary

Run the full payment sandwich flow (S1→S2→S6→S3→S4→S5→S7) against real sandbox APIs.

Acceptance Criteria

• Happy-path payment completes: initiate → compliance → FX quote → fiat collection → blockchain transfer → USDC redemption → SEPA payout → ledger entries
• At least 1 failure path (compliance rejection) works
• Webhooks received from Stripe + Modulr sandboxes
• Ledger entries balanced (debits = credits)
• Document results in docs/sandbox-test-results.md

Depends On

• STA-213 (sandbox profiles for all services)
• All provider sandbox adapters (STA-203 through STA-211)

🤖 Generated with Claude Code

[Puneethkumarck]

First real E2E sandbox payment completed — $5.00 USD → £3.73 GBP through all 7 services with real sandbox APIs (Persona, Frankfurter, Stripe, Alchemy/Base Sepolia, Circle, Modulr). 16 bugs found and fixed. PR #248.