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/handlers/imageenc.go

@@ -66,6 +66,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 
 			return
 		}
+
 		defer func() { _ = resp.Content.Close() }()
 
 		// Set response headers