feat: add getInvoiceAge and getFundingVelocity to invoiceStats (Closes #611)

[waterWang]

Closes #611

Summary

Adds getInvoiceAge and getFundingVelocity helper functions to src/invoiceStats.ts for computing derived invoice metrics from existing Invoice fields without network calls.

Changes

src/invoiceStats.ts

• getInvoiceAge(invoice): Returns { days, hours, minutes } since createdAt. Uses Date.now() internally.
• getFundingVelocity(invoice): Returns USDC funded per day since creation. Returns 0 if createdAt is within the same second as Date.now() (prevents division by zero).
• Both handle createdAt as either Unix timestamp (seconds) or milliseconds — auto-detected by magnitude (> 1e12 = ms).

Supporting changes

• src/types.ts: Added createdAt: number to Invoice type
• src/client.ts: Updated _parseInvoice to parse createdAt from raw contract data
• src/testing/factories.ts and src/testing/mockFactory.ts: Added createdAt to mock invoice factories
• src/index.ts: Public API export

Tests

• test/invoiceStats.test.ts: 7 unit tests covering age, velocity, zero-age guard, and timestamp format auto-detection

All 7 tests pass.