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/service.go

@@ -91,15 +91,17 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 	if result != nil && result.Hit {
 		s.cache.IncrementStats(ctx, true, 0)
 
-		reader, err := s.cache.GetOutput(result.OutputHash)
+		reader, size, err := s.cache.GetOutputWithSize(result.OutputHash)
 		if err != nil {
 			s.log.Error("failed to get cached output", "hash", result.OutputHash, "error", err)
 			// Fall through to re-fetch
 		} else {
 			return &ImageResponse{
-				Content:     reader,
-				ContentType: result.ContentType,
-				CacheStatus: CacheHit,
+				Content:       reader,
+				ContentLength: size,
+				ContentType:   result.ContentType,
+				CacheStatus:   CacheHit,
+				ETag:          formatETag(result.OutputHash),
 			}, nil
 		}
 	}
@@ -173,15 +175,17 @@ func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*Imag
 	}
 
 	// Serve from the cached file on disk (same path as cache hits)
-	reader, err := s.cache.GetOutput(outputHash)
+	reader, size, err := s.cache.GetOutputWithSize(outputHash)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read cached output: %w", err)
 	}
 
 	return &ImageResponse{
-		Content:      reader,
-		ContentType:  processResult.ContentType,
-		FetchedBytes: int64(len(sourceData)),
+		Content:       reader,
+		ContentLength: size,
+		ContentType:   processResult.ContentType,
+		FetchedBytes:  int64(len(sourceData)),
+		ETag:          formatETag(outputHash),
 	}, nil
 }
 
@@ -260,3 +264,16 @@ func extractStatusCode(err error) int {
 
 	return httpStatusInternalError
 }
+
+// etagHashLength is the number of hash characters to use for ETags.
+const etagHashLength = 16
+
+// formatETag formats a hash as a quoted ETag value.
+func formatETag(hash string) string {
+	// Use first 16 characters of hash for a shorter but still unique ETag
+	if len(hash) > etagHashLength {
+		hash = hash[:etagHashLength]
+	}
+
+	return `"` + hash + `"`
+}