fix: correct the type errors introduced in #1290

[SagaciousBoss]

Follow-up to #1290, which merged with four type errors that only the Docker build job (tsc) caught — the backend CI jobs run eslint and vitest, neither of which type-checks. #1290 passed Backend (Node.js 20) and Backend (Node.js 22).

What was wrong

• backend/src/services/fiat-crypto.ts — const conversions = Map<string, ConversionRecord>() was missing new. That also made Array.from(conversions.values()) an unknown[], which broke the element types in listConversions.
• backend/src/services/payouts/payout-engine.ts — evaluatePayouts called the async materialiseBatch without awaiting it, so it pushed a Promise into a PayoutBatch[], and the per-schedule try/catch could not observe a failure.
• backend/src/routes/payouts.ts — req.params.batchId is typed string | string[] under this Express version. The handlers now resolve the id once through requireId and reuse the validated string, which also removes the repeated param lookup.

Two smaller robustness changes while in the same code:

• The FX rate lookup and the payout asset-scale lookup use Object.prototype.hasOwnProperty.call rather than comparing the result to undefined, which keeps them honest if a rate is ever legitimately falsy.
• The settlement amount parse moved into parseSettlementAmount, which collapses every failure mode onto the single INVALID_SETTLEMENT_AMOUNT sentinel instead of relying on definite-assignment analysis across a try/catch.

Scope

Three files, no behaviour change beyond fixing the above. Backend (Node.js 20/22) on #1290 passed, so the test suite already covered this code; this commit makes tsc agree.

Verification

npm run test --workspace backend and npm run lint --workspace backend. The repository-wide tsc build has a large number of pre-existing errors unrelated to this change (missing modules, ungenerated Prisma client, websocket.ts, tax-engine.ts, featureFlagRegistry.ts and others); this commit removes the only ones introduced by #1290.

Closes #1184
Closes #1185
Closes #1186

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