Use original publication date for post createdAt

Posts now use the item pub_date for the createdAt field instead
of the current time, so posts show their original publication time.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

publisher.go

@@ -346,12 +346,11 @@ func (p *Publisher) PublishItem(session *PDSSession, item *Item) (string, error)
 	}
 	text := textBuilder.String()
 
-	// Use current time for createdAt (Bluesky won't index backdated posts)
-	// TODO: Restore original pubDate once Bluesky indexing is understood
+	// Use original publication date if available, otherwise current time
 	createdAt := time.Now()
-	// if !item.PubDate.IsZero() {
-	// 	createdAt = item.PubDate
-	// }
+	if !item.PubDate.IsZero() {
+		createdAt = item.PubDate
+	}
 
 	post := BskyPost{
 		Type:      "app.bsky.feed.post",