feat(wallet): implement Wallet clone with atomic transfers and ledger

[iQodeIT]

What this does

Implements the Wallet clone (Bounty #6) with atomic transfers and an append-only transaction ledger.

The invariant

Satisfies the "never negative balance" and "atomic transfer" invariants. Transfers are executed within a database transaction, ensuring that a debit only happens if the balance is sufficient, and the corresponding credit and ledger entry are created atomically. Verified in apps/api/test/wallet.test.ts.

How I verified it

Ran the local probe:

node probes/wallet.mjs http://localhost:3000

Result: All transfers succeeded or failed as expected under concurrency, with final balances matching the ledger.

Checklist

• Whole test suite is green from a clean checkout (pnpm -w run test)
• The concurrency/correctness probe passes with zero 5xx
• Migrations apply cleanly on a fresh DB; key events are logged
• No regressions to other clones' probes
• I've read TERMS.md

AI usage (directed vs. decided):
Directed the AI to implement the Prisma schema and the Fastify routes for the wallet system, including concurrency handling using database transactions. Decided on the specific error handling and validation logic.