feat(pricing): enforce quote amount bounds

[changliuchang777]

Summary

• add BigInt-based quote bound checks for per-pair minAmount, maxAmount, and liquidity metadata
• return canonical single-quote errors for below-min/above-max and insufficient liquidity
• report bulk quote bound failures per item without failing the whole batch
• document bound semantics and the zero-means-unset rule

Fixes #2

Validation

• PASS: git diff --check
• BLOCKED: npm test -- --runInBand src/tests/index.test.ts -t "quote amount bounds" fails before running tests because main currently has duplicate declarations in src/index.ts: WEBHOOK_MAX_EVENTS is already declared
• BLOCKED: npx eslint src/index.ts --quiet fails on existing main unused variables: WebhookRecord, PORT, isKeyValid, requireScope, resolvePair, computeQuoteTimes

Precision strategy

All quote bound comparisons parse metadata strings and request amounts as BigInt. The implementation never coerces base-unit amounts through Number, and treats metadata value "0" as an unset bound.