feat(cms-images): forward dpr + format=auto to RustyCMS

- Accept dpr query param (clamped 1..4) and include it in cache key so
  each density gets its own cached file.
- Accept format=auto and negotiate locally from the client's Accept
  header (avif > webp > jpeg) before building the cache key — concrete
  formats in the cache avoid cross-client leaks.

src/lib/image-cache.server.ts

@@ -29,6 +29,7 @@ export interface ImageTransformParams {
   ar?: string;
   fx?: number;
   fy?: number;
+  dpr?: number;
 }
 
 const memoryCache = new Map<string, CachedImage>();
@@ -60,6 +61,7 @@ export function buildCacheKey(src: string, params: ImageTransformParams): string
   if (params.ar) parts.push(`ar=${params.ar}`);
   if (params.fx != null) parts.push(`fx=${roundFocal(params.fx)}`);
   if (params.fy != null) parts.push(`fy=${roundFocal(params.fy)}`);
+  if (params.dpr != null) parts.push(`dpr=${roundFocal(params.dpr)}`);
   return createHash('sha256').update(parts.join('|')).digest('hex').slice(0, 16);
 }