Spec #136: finished becomes a Series fact, Lane gate rewritten on it (#157)

finished_at lands on series (0016 seeds it from the pre-flip finished
bookmarks, then flips those bookmarks to archived), the poll gate and
eligible count read the flag instead of a per-Reader vote, PUT rejects
the finished status like any unknown value, and the web UI drops the
Finished tab, badge and finish button. Novel merge rank is archived >
reading. Per-Reader disagreement (one Reader keeps a finished Series in
reading forever) is what the flag repairs; the cutover keeps polling
state unchanged for every Series.

backend/main_test.go

@@ -78,7 +78,7 @@ func TestPutStatusValidation(t *testing.T) {
 		{"empty is no opinion", "", http.StatusOK},
 		{"reading", "reading", http.StatusOK},
 		{"archived", "archived", http.StatusOK},
-		{"finished is web-only", "finished", http.StatusBadRequest},
+		{"finished is no longer a bucket", "finished", http.StatusBadRequest},
 		{"garbage", "dropped", http.StatusBadRequest},
 	}
 	for _, tc := range cases {