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/storage.go

@@ -114,6 +114,29 @@ func (s *ContentStorage) Load(hash string) (io.ReadCloser, error) {
 	return f, nil
 }
 
+// LoadWithSize returns a reader and file size for the content with the given hash.
+func (s *ContentStorage) LoadWithSize(hash string) (io.ReadCloser, int64, error) {
+	path := s.hashToPath(hash)
+
+	f, err := os.Open(path) //nolint:gosec // content-addressable path from hash
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, 0, ErrNotFound
+		}
+
+		return nil, 0, fmt.Errorf("failed to open content: %w", err)
+	}
+
+	stat, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+
+		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
+	}
+
+	return f, stat.Size(), nil
+}
+
 // Delete removes content with the given hash.
 func (s *ContentStorage) Delete(hash string) error {
 	path := s.hashToPath(hash)