Refactor to serve all responses from cached files on disk

- StoreOutput now returns output hash for immediate retrieval
- Cache misses now serve from disk file after storing (same as hits)
- Log served_bytes from actual io.Copy result (avoids stat calls)
- Remove ContentLength field usage for cache hits (stream from file)
- Fix tests to properly check all return values

internal/imgcache/service.go

@@ -88,10 +88,9 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 			// Fall through to re-fetch
 		} else {
 			return &ImageResponse{
-				Content:       reader,
-				ContentLength: -1, // Unknown until read
-				ContentType:   result.ContentType,
-				CacheStatus:   CacheHit,
+				Content:     reader,
+				ContentType: result.ContentType,
+				CacheStatus: CacheHit,
 			}, nil
 		}
 	}
@@ -152,27 +151,28 @@ func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*Imag
 		return nil, fmt.Errorf("image processing failed: %w", err)
 	}
 
-	// Read processed data to cache it
-	processedData, err := io.ReadAll(processResult.Content)
-	if err != nil {
-		return nil, fmt.Errorf("failed to read processed image: %w", err)
-	}
-	_ = processResult.Content.Close()
-
-	// Store output content
+	// Store output content to cache
 	metaID, err := s.cache.GetSourceMetadataID(ctx, req)
-	if err == nil {
-		err = s.cache.StoreOutput(ctx, req, metaID, bytes.NewReader(processedData), processResult.ContentType)
-		if err != nil {
-			s.log.Warn("failed to store output content", "error", err)
-		}
+	if err != nil {
+		return nil, fmt.Errorf("failed to get source metadata ID: %w", err)
+	}
+
+	outputHash, err := s.cache.StoreOutput(ctx, req, metaID, processResult.Content, processResult.ContentType)
+	_ = processResult.Content.Close()
+	if err != nil {
+		return nil, fmt.Errorf("failed to store output content: %w", err)
+	}
+
+	// Serve from the cached file on disk (same path as cache hits)
+	reader, err := s.cache.GetOutput(outputHash)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read cached output: %w", err)
 	}
 
 	return &ImageResponse{
-		Content:       io.NopCloser(bytes.NewReader(processedData)),
-		ContentLength: int64(len(processedData)),
-		ContentType:   processResult.ContentType,
-		FetchedBytes:  int64(len(sourceData)),
+		Content:      reader,
+		ContentType:  processResult.ContentType,
+		FetchedBytes: int64(len(sourceData)),
 	}, nil
 }