Owner notices: the stall, one webhook path from env to Discord (#171)

Rollout note: the owner_notices table starts empty, so the first pass after deploy sends for conditions already true — correct per one-row-per-episode; say so rather than have it reported as a bug. Prod step: create the webhook, set DISCORD_WEBHOOK_URL on the deployment, redeploy — unset is silent by design, and without that step the feature ships dark. Security invariants preserved: the webhook address is a secret in the class of TOKEN_KEY (never logged, never on a config-printing line), and the owner gate is unchanged.

backend/internal/store/store_test.go

@@ -2700,3 +2700,41 @@ func failureRow(t *testing.T, s *Store, site, seriesID string) (word string, sin
 	}
 	return word, since, true
 }
+
+// Owner notices are one row per episode: the row's presence is the whole
+// state. MarkNoticeSent stamps it, NoticeSent reads it, and ClearNotice
+// forgets it — including a row that was never there, which is not an error.
+func TestOwnerNoticeRows(t *testing.T) {
+	s := newTestStore(t)
+
+	if sent, err := s.NoticeSent("stall", "comix"); err != nil || sent {
+		t.Fatalf("NoticeSent on a fresh table = %v, %v; want false, nil", sent, err)
+	}
+
+	if err := s.MarkNoticeSent("stall", "comix", 4242); err != nil {
+		t.Fatalf("MarkNoticeSent: %v", err)
+	}
+	if sent, err := s.NoticeSent("stall", "comix"); err != nil || !sent {
+		t.Fatalf("NoticeSent after mark = %v, %v; want true, nil", sent, err)
+	}
+
+	// Re-marking the same episode just moves the stamp: the row is a lock,
+	// not a log. A different site is its own episode — the key is
+	// (condition, site).
+	if err := s.MarkNoticeSent("stall", "comix", 4343); err != nil {
+		t.Fatalf("re-mark: %v", err)
+	}
+	if sent, err := s.NoticeSent("stall", "kagane"); err != nil || sent {
+		t.Fatalf("NoticeSent on another site = %v, %v; want false, nil", sent, err)
+	}
+
+	if err := s.ClearNotice("stall", "comix"); err != nil {
+		t.Fatalf("ClearNotice: %v", err)
+	}
+	if sent, err := s.NoticeSent("stall", "comix"); err != nil || sent {
+		t.Fatalf("NoticeSent after clear = %v, %v; want false, nil", sent, err)
+	}
+	if err := s.ClearNotice("stall", "comix"); err != nil {
+		t.Fatalf("ClearNotice on a missing row: %v", err)
+	}
+}