Serve a cached variant while its source URL is negative-cached (closes #245)

Get checked the negative cache before it looked for the variant on
disk, so after one failed fetch of a source every cached variant of it
answered 502 for 5 minutes. Get now serves a cached variant first and
checks the negative cache only on a miss, before processing. The 502
line under /v1/image/ in README.md and TODO.md say so.

Model: opus-5-5

internal/imgcache/service.go

@@ -129,26 +129,13 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 // ErrNegativeCached is returned when a URL is in the negative cache (recently failed).
 var ErrNegativeCached = errors.New("request is in negative cache (recently failed)")
 
-// Get retrieves a processed image, fetching and processing if necessary.
+// Get retrieves a processed image, fetching and processing if necessary. A
+// cached variant is served even while its source URL is in the negative
+// cache, which refuses only a variant that is not cached.
 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 {
@@ -175,6 +162,21 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		}
 	}
 
+	// 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)
+	}
+
 	// Cache miss - get the variant, processed once for all the requests that
 	// miss it at the same time, then count this request's miss, also when it
 	// failed or the request context has ended meanwhile