feat(web): stamp login_events on every gated request

The registration gate, once it resolves the authenticated subject to a tapir
user_id, calls StampLogin (store-throttled to one row per user per day). Best-
effort: a stamp failure is logged and swallowed so it never breaks the request.
This is what makes the read-side Stage-0 usage signal actually accrue.

Tests cover the happy-path stamp, the same-day throttle, and that an
unregistered subject (redirected to /register) is never stamped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/registration.go

@@ -64,6 +64,13 @@ func (a *App) registrationGate(h http.Handler) http.Handler {
 			http.Redirect(w, r, registerPath, http.StatusFound)
 			return
 		}
+		// Stamp the read-side usage signal (Stage-0 gate, ADR-016): the store
+		// throttles this to one row per user per day, so a stamp on every gated
+		// request is cheap. Best-effort — a stamp failure must never break the
+		// request the user actually came for, so it is logged and swallowed.
+		if err := a.Store.StampLogin(r.Context(), userID); err != nil {
+			a.logger().Warn("stamp login event", "user", userID, "err", err)
+		}
 		h.ServeHTTP(w, r.WithContext(withUserID(r.Context(), userID)))
 	})
 }