feat: WebP output format

[ajianaz]

Demand source: Bannerbear, Robolly, every image API offers WebP. ~30% smaller than PNG for screenshots/flat graphics.

Proposed design:

• CLI: --format webp|png (default png).
• API: Accept: image/webp or a response_format extension.
• resvg encode: check whether resvg -> tiny-skia can encode WebP directly; if not, encode via the image crate (already in the tree via feat(render): user-positioned background images (zoom + focal point) #82) with default quality 85.

Acceptance:

• Valid WebP output (RIFF....WEBP magic bytes) and smaller than the equivalent PNG.
• PNG default stays backward compatible.

Effort: S. Priority #3.