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/httpfetcher/httpfetcher.go

@@ -198,6 +198,7 @@ func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 
 	// If we fail before returning a result, release the slot
 	success := false
+
 	defer func() {
 		if !success {
 			<-sem