Allow itemized invoices for multiple PR fixes

[phucnguyen1707]

Summary

• allow fixed/bounty code gigs to submit one itemized invoice for multiple PR fixes when enough direct PR links back the billed units
• keep the agreed-amount cap for non-code invoices, flat over-cap invoices, overpriced PR units, or invoices without enough unique PR links
• include both top-level and line-item PR links in invoice metadata/emails so posters can verify all merged work
• mirror the same validation in the invoice form so the UI does not block valid multi-PR code invoices

Why

The current fixed-price cap treats a $1 code gig as a single total payout, even when the work agreement is effectively $1 per merged PR. That prevents workers from submitting one clean invoice for 2+ merged PRs and forces serial revoke/resend invoice flows.

Guardrails

• Only applies to category=code invoices with itemized line items
• Each billed unit must be at or below the agreed rate
• The invoice needs at least as many unique direct GitHub PR URLs as billed units
• GitHub owner/repo casing is normalized before dedupe so the same PR cannot be counted twice with different casing

Tests

• pnpm exec vitest run 'src/app/api/gigs/[id]/invoice/route.test.ts' src/lib/github-links.test.ts
• pnpm run type-check
• pnpm exec eslint 'src/app/api/gigs/[id]/invoice/route.ts' 'src/app/api/gigs/[id]/invoice/route.test.ts' src/components/gigs/InvoiceButton.tsx
• pnpm run build
• pre-commit: full lint + type-check + test:run + build