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/handlers/image.go

@@ -124,26 +124,26 @@ func (s *Handlers) HandleImage() http.HandlerFunc {
 			w.Header().Set("ETag", resp.ETag)
 		}
 
-		// Log cache status and timing
+		// Stream the response
+		w.WriteHeader(http.StatusOK)
+
+		servedBytes, err := io.Copy(w, resp.Content)
+		if err != nil {
+			s.log.Error("failed to write response",
+				"error", err,
+			)
+		}
+
+		// Log cache status and timing after serving
 		duration := time.Since(startTime)
 		s.log.Info("image served",
 			"cache_key", cacheKey,
 			"cache_status", resp.CacheStatus,
 			"duration_ms", duration.Milliseconds(),
 			"format", req.Format,
-			"served_bytes", resp.ContentLength,
+			"served_bytes", servedBytes,
 			"fetched_bytes", resp.FetchedBytes,
 		)
-
-		// Stream the response
-		w.WriteHeader(http.StatusOK)
-
-		_, err = io.Copy(w, resp.Content)
-		if err != nil {
-			s.log.Error("failed to write response",
-				"error", err,
-			)
-		}
 	}
 }