[codex] validate fund invoice stroops as BigInt strings

[MyouzzZ]

Closes #506

Summary

• Reuse the shared validateAmountStroops BigInt validator in POST /api/invest/fund-invoice instead of coercing through Number().
• Keep amountStroops as a strict decimal string through the funding path and reject JSON numbers, signs, decimals, scientific notation, leading zeros, zero, and values above 10^18 before escrow submission or persistence.
• Validate the escrow submission entry point as well so stubbed/delegated/submitted paths never accept a JS number.
• Document the string-only amountStroops contract in README and the route OpenAPI block, with focused route/service tests.
• Restore small broken support exports/imports/metrics/logger wrapping that prevented the relevant route/CI tests from loading cleanly in this checkout.

Tests

• npm test -- --silent tests/invest.fund.test.js tests/investorCommitment.validation.test.js tests/escrowSubmit.amount.test.js tests/escrow.derived.test.js
• npx eslint src/routes/invest.js src/services/investorCommitment.js src/services/escrowSubmit.js src/middleware/kycGating.js src/middleware/requestId.js src/middleware/correlationId.js src/metrics.js src/logger.js tests/invest.fund.test.js tests/escrowSubmit.amount.test.js