docs: drop the last references to cover scraping (#60)

Review follow-ups. The README's adapter reference and the userscript
testing skill still described a scrape that no longer exists, and the
manga test stub kept a querySelectorAll whose only caller was the
deleted coverFromPage. The handler comment's premise ("every installed
userscript still sends one") stops being true the moment a Reader
reinstalls, so it now says older copies may.

backend/internal/api/handlers.go

@@ -51,11 +51,11 @@ func (h *Handler) Put(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	// A body may carry a cover, and it is discarded here rather than
-	// rejected: every installed userscript still sends one, and ADR-0004's
-	// compatibility argument depends on those scripts continuing to work. The
-	// Cover is acquired server-side (ADR-0007), so the field is permanently
-	// inert - not pending removal, and not a value any later code should
-	// start reading.
+	// rejected: an older installed userscript may still send one, and
+	// ADR-0004's compatibility argument depends on those scripts continuing
+	// to work. The Cover is acquired server-side (ADR-0007), so the field is
+	// permanently inert - not pending removal, and not a value any later code
+	// should start reading.
 	b.Cover = ""
 
 	// Path key is authoritative; derive site/series_id from it when the body