feat(api): auto-generated Open Graph images for links + invoices (#165)

[rahXephonz]

Implements RFC #165.

What

Render a per-document 1200×630 PNG preview card for payment links and
invoices, and inject the matching og:image / twitter:image meta tags into
the checkout and invoice page shells. A pay-testnet.mayarin.xyz/checkout/:id
or /invoices/:id/view link shared in WhatsApp / Telegram / X now unfurls with
the merchant name and the human-form amount instead of the generic landing
image.

How

• apps/api/src/services/og-image.ts — satori (element tree → SVG) +
  @resvg/resvg-js (SVG → PNG). One template, brand palette lifted from
  apps/checkout-ui/src/styles.css. Money is rendered with formatMoneyLocale
  (the id-ID human form) — never toDecimalString, never a number.
• Fonts — static Geist instances (geist-regular/semibold/bold.ttf) bundled
  in apps/api/assets/. The variable TTF chokes satori's bundled opentype
  parser on the fvar axes, so static instances are required. OFL license
  included.
• Routes — GET /checkout/:id/og.png and GET /invoices/:id/og.png.
  Missing / disabled / voided document or any render failure → cached generic
  card, 200, never 500 (a shared link never loses its preview card).
  Cache-Control: public, max-age=86400 (link content is immutable after
  create).
• Shell meta — checkout-shell.ts injects per-document OG meta via a new
  <!--__OG__--> placeholder (alongside <!--__BOOTSTRAP__-->). Link + invoice
  pages build it from the document; the pay page keeps the generic landing image
  (its intent id is unguessable, not a shared URL).
• og:image URL — same-origin via requestOrigin (RFC RFC: Dedicated checkout host — pay.mayarin.xyz (Phase 4) #163), so a pay-host
  link points at pay-testnet.../checkout/:id/og.png through the proxy.
• Pay proxy — allowlist forwards /invoices/:id/og.png (the
  /checkout/:id/og.png is already covered by the /checkout prefix).

Cards

• Fixed / catalog link → merchant name + formatMoneyLocale(total) + asset code.
• Open link (no amount) → title, or Pay {merchant} when it has none.
• Invoice → merchant name + Invoice {number} + outstanding total.
• Settled invoice → Settled state label + total. Voided → Voided label.

Verification

• bun run typecheck — green.
• bun test — 1329 pass, 0 fail. New tests: og-image.test.ts (card + meta
  builders, pure) and checkout-shell.test.ts (/checkout/:id/og.png renders a
  real PNG, missing link → 200 generic, page shells carry the right og:image).
• apps/checkout-ui build preserves both placeholders in dist/index.html.

Out of scope (per RFC #165 non-goals)

AI image generation, per-merchant branded backgrounds, merchant logo, video
previews, mainnet hosting, OG for landing/dashboard/demo.

Closes #165.