OG image endpoints fetch font from a third-party URL at build time #13

Describe the bug

Both OG image endpoints (`src/pages/v1/generate/og/[slug].png.ts` and `default.png.ts`) did a top-level `await fetch(...)` against a third-party demo host (`og-playground.vercel.app`) to get a font at build time. Every production build depends on that unrelated service's uptime — a network call that has no business being part of the build.

Expected behavior

No network calls at build time. The font should be vendored into the repo (`public/fonts/`) and read via `readFileSync`.

Additional context

The host was reachable throughout, so this was never actually the cause of an outage — but it was a real latent fragility, and the code already carried a `@TODO` asking for the font to be vendored. Fixed in #9.