Perf: serve WebP/AVIF via Vercel Image Optimization (HITL)

[wab]

Parent

#120

What to build

Configure <OptimizedImage> (from #151) to emit Vercel Image Optimization URLs so browsers receive WebP/AVIF where supported. Image payload target: -40%.

HITL: requires verification of the Vercel plan's Image Optimization quota — if limited, decide between Cloudinary or build-time optimization fallback.

Acceptance criteria

• Vercel plan quota verified, decision recorded in issue comments
• <OptimizedImage> routes through /_vercel/image (or chosen alternative)
• WebP / AVIF served per Accept header
• Image payload reduction measured on a representative listing page
• Fallback path for unoptimized images documented

Blocked by

• Perf: <OptimizedImage> component + hero LCP wiring #151