refactor(reader): use time.Duration instead of minutes count

In general, duration is used as time unit representation.

At some places when int is returned, there's no documentation which unit is used.

So just convert to time.Duration ASAP.

internal/reader/rss/adapter.go

@@ -55,7 +55,7 @@ func (r *rssAdapter) buildFeed(baseURL string) *model.Feed {
 	// Get TTL if defined.
 	if r.rss.Channel.TTL != "" {
 		if ttl, err := strconv.Atoi(r.rss.Channel.TTL); err == nil {
-			feed.TTL = ttl
+			feed.TTL = time.Duration(ttl) * time.Minute
 		}
 	}