Implement ETag, HEAD requests, and conditional requests

- Add ETag generation based on output content hash (first 16 chars)
- Add ContentLength to ImageResponse from cache
- Add LoadWithSize method to ContentStorage
- Add GetOutputWithSize method to Cache
- Handle HEAD requests returning headers only
- Handle If-None-Match conditional requests returning 304
- Register HEAD route for image proxy endpoint

internal/imgcache/cache.go

@@ -168,6 +168,11 @@ func (c *Cache) GetOutput(outputHash string) (io.ReadCloser, error) {
 	return c.dstContent.Load(outputHash)
 }
 
+// GetOutputWithSize returns a reader and size for cached output content.
+func (c *Cache) GetOutputWithSize(outputHash string) (io.ReadCloser, int64, error) {
+	return c.dstContent.LoadWithSize(outputHash)
+}
+
 // StoreSource stores fetched source content and metadata.
 func (c *Cache) StoreSource(
 	ctx context.Context,