Adventure: stop signing my own posts

The source tag credits an outlet Pete is relaying (`ars technica`). On
his own reporting it rendered as a trailing `pete` under a message he
already sent - him signing his own name. Drop Source on adventure posts
and omit the tag line entirely when there's nothing to credit; RSS posts
keep theirs.

internal/web/adventure.go

@@ -151,12 +151,14 @@ func (s *Server) handleAdventureIngest(w http.ResponseWriter, r *http.Request) {
 	if f.Tier == "priority" && s.advPost != nil && s.adv.Channel != "" {
 		// No ImageURL: the emblem is an SVG (Matrix clients often block SVG
 		// media), and the link's og:image carries the preview instead.
+		// No Source: the source tag exists to credit an outlet Pete is relaying
+		// (`ars technica`). On his own reporting it renders as him signing his
+		// own name under his own message.
 		s.advPost(AdvPost{
 			GUID:       f.GUID,
 			Headline:   headline,
 			Lede:       lede,
 			ArticleURL: articleURL,
-			Source:     advSource,
 			Channel:    s.adv.Channel,
 		})
 	}