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/handlers/image.go

@@ -122,6 +122,22 @@ func (s *Handlers) HandleImage() http.HandlerFunc {
 
 		if resp.ETag != "" {
 			w.Header().Set("ETag", resp.ETag)
+
+			// Check for conditional request (If-None-Match)
+			if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
+				if ifNoneMatch == resp.ETag {
+					w.WriteHeader(http.StatusNotModified)
+
+					return
+				}
+			}
+		}
+
+		// Handle HEAD request - return headers only
+		if r.Method == http.MethodHead {
+			w.WriteHeader(http.StatusOK)
+
+			return
 		}
 
 		// Stream the response