style: satisfy wsl_v5 blank-line-before-defer rule

Insert the blank line wsl_v5 requires above four defer statements
whose deferred closure does not share a variable with the immediately
preceding line, across internal/handlers, internal/httpfetcher, and
internal/imgcache. No behavior change.

internal/imgcache/service.go

@@ -315,6 +315,7 @@ func (s *Service) fetchAndProcess(
 
 		return nil, fmt.Errorf("upstream fetch failed: %w", err)
 	}
+
 	defer func() { _ = fetchResult.Content.Close() }()
 
 	// Read and validate the source content