feat: batch OG image generation API (v0.6.0)

Add POST /v1/og/batch endpoint for generating up to 20 images in one
request. Supports defaults merging, parallel rendering via
Promise.allSettled, base64-encoded JSON responses, and quota pre-check.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

feat: custom font URL support (v0.5.0)

Add `fontUrl` parameter to load custom TTF/OTF fonts from any URL.
Fonts are validated by magic bytes, cached via Workers Cache API (1h TTL),
and gracefully fallback to Inter on any error.

- New param: fontUrl (URL to .ttf or .otf, max 5MB)
- Cache layer: Workers Cache API with 1h TTL
- Fallback: X-Font-Fallback response header when using default font
- Landing page: custom fonts section + playground font URL field

docs: add comprehensive README and MIT license

README includes quick start guides, template previews, full API reference,
SDK examples, framework integration examples (Next.js, Astro, Hugo),
self-hosting instructions, and pricing table.

Co-Authored-By: Claude Opus 4.6 <[email protected]>