feat: a Reader's Sighting defers a Poll of a solitary Series (#103)

A userscript PUT already carries the Latest Chapter the Reader's own browser
read; it now stands in for a Poll where being wrong can hurt nobody else.
Deferral lives in the due query beside the rest cutoff: one Bookmark, sighted
within one rest, under the six-rest ceiling. checkOne judges the Reader whose
report raised the value off the comparison it already makes - three
contradictions stop them deferring, twenty confirmations forgive.

ADR-0011 records the trust model and the rejected alternatives.

backend/internal/api/handlers.go

@@ -99,7 +99,18 @@ func (h *Handler) Put(w http.ResponseWriter, r *http.Request) {
 	// reading progress actually moved. Any client value is ignored.
 	b.UpdatedAt = time.Now().UnixMilli()
 
-	stored, err := h.Store.Upsert(httpmw.ReaderID(r), b)
+	// A userscript PUT is a Sighting: the Reader's browser was on the Series
+	// page and read its Latest Chapter (issue #103). Recorded before the
+	// Upsert, which is what makes the raise comparison possible, and never
+	// from the web UI's own read-modify-write — a Reader toggling a favourite
+	// has not looked at the Site and must not postpone a Poll. A failure here
+	// costs a deferral, not the write, so it is logged and dropped.
+	readerID := httpmw.ReaderID(r)
+	if err := h.Store.RecordSighting(readerID, b.Site, b.SeriesID, b.LatestChapterNum, b.UpdatedAt); err != nil {
+		log.Printf("record sighting: %v", err)
+	}
+
+	stored, err := h.Store.Upsert(readerID, b)
 	if err != nil {
 		log.Printf("upsert: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)