Serve a cached variant while its source URL is in the negative cache (closes #245)

Get checked the negative cache before the variant cache, so after one
failed fetch of a source every variant of it answered 502 for five
minutes, also one on disk or one that could be made from the cached
source. The check now sits at the start of fetchAndProcess, which runs
only when no cached source can be used. A refusal from the negative
cache now counts as a miss.

Model: opus-5-5

internal/imgcache/service.go

@@ -130,25 +130,13 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 var ErrNegativeCached = errors.New("request is in negative cache (recently failed)")
 
 // Get retrieves a processed image, fetching and processing if necessary.
+// While its source URL is in the negative cache, a cached variant is still
+// served and a variant not in the cache is still made from a cached source;
+// only a request that needs the source fetched is refused.
 func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, error) {
 	// Propagate AllowHTTP setting to the request
 	req.AllowHTTP = s.allowHTTP
 
-	// Check negative cache first - skip fetching for recently-failed URLs
-	negHit, err := s.cache.checkNegativeCache(ctx, req)
-	if err != nil {
-		s.log.Warn("negative cache check failed", "error", err)
-	}
-
-	if negHit {
-		s.log.Debug("negative cache hit",
-			"host", req.SourceHost,
-			"path", req.SourcePath,
-		)
-
-		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
-	}
-
 	// Check variant cache first (disk only, no DB)
 	result, err := s.cache.Lookup(ctx, req)
 	if err != nil {
@@ -433,12 +421,28 @@ func (s *Service) processFromSourceOrFetch(
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
 // It counts the fetch with the bytes read from upstream, including when
-// reading the response or a later step fails.
+// reading the response or a later step fails. It fetches nothing while the
+// source URL is in the negative cache.
 func (s *Service) fetchAndProcess(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
 ) (*processedVariant, error) {
+	// Check negative cache - skip fetching for recently-failed URLs
+	negHit, err := s.cache.checkNegativeCache(ctx, req)
+	if err != nil {
+		s.log.Warn("negative cache check failed", "error", err)
+	}
+
+	if negHit {
+		s.log.Debug("negative cache hit",
+			"host", req.SourceHost,
+			"path", req.SourcePath,
+		)
+
+		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
+	}
+
 	// Fetch from upstream
 	sourceURL := req.SourceURL()