Add PDF support to image transforms (setTransform) and GraphQL

[Megafry]

PDF assets are common (downloads, job applications, document libraries), but they can't use Craft's transform system today. setTransform() / getUrl() only work for image types — PDFs are excluded from canManipulateAsImage().
It would be useful if PDFs could be treated like other assets for transforms: pick a page, resize, and output as webp / jpg / png via the same API used everywhere else.

Proposed behaviour

• {% do asset.setTransform({ width: 800, format: 'webp' }) %} works on PDF assets
• asset.url, getSrcset(), etc. return rasterized previews (first page by default)
• GraphQL transforms work the same way (@transform on PDF assets)
• Opt-in globally via config (e.g. transformPdfAssets: true), since it depends on Imagick/Ghostscript

Use case

Show a WebP preview/thumbnail of a PDF on the front end without a separate plugin or custom Twig filter.