feat: add CMS image optimizer

[glitchedmob]

Adds a narrow SvelteKit replacement for the Next image optimizer used by CMS media while keeping future CMS updates dynamic.

• Adds /_image for trusted /cms-media/media/... images with Next-compatible width and quality validation, Sharp transforms, WebP negotiation, ETags, HEAD support, and bounded upstream fetches.
• Adds CmsImage.svelte plus helper functions that emit the fixed-width 1x/2x srcset shape Next used.
• Pins sharp to 0.35.4 to match the original Next app lock.
• Extends unit and adapter-node smoke tests for optimizer output, URL validation, cache headers, conditional requests, slash redirects, and no-enlarge behavior.