feat(api): read-all takes an optional up_to cutoff (M489)

POST /api/me/notifications/read-all accepts {"up_to": RFC3339}. Android
queues "mark all read" for replay when offline, and a replay landing later
must not mark notices that arrived in between, which the user never saw.
A coalesced notice updated since then has a newer created_at, so it stays
unread. An empty body still marks everything.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/me_notifications_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -127,6 +128,33 @@ func TestMyNotifications_MarkReadIsOwnerScopedAndCountsDown(t *testing.T) {
 	}
 }
 
+// A "mark all read" replayed from an offline queue carries the moment the
+// user asked; what arrived after it stays unread.
+func TestMyNotifications_ReadAllUpToLeavesLaterOnesUnread(t *testing.T) {
+	h, pool := testHandlers(t)
+	alice := seedUser(t, pool, "notif-upto", "pw", false)
+	r := notificationsRouter(h)
+
+	notifyN(t, pool, alice, 1)
+	var page notificationsPageResp
+	callAs(t, r, alice, http.MethodGet, "/api/me/notifications", "", &page)
+	seen := page.Items[0].CreatedAt
+	notifyN(t, pool, alice, 1)
+
+	body := `{"up_to":"` + seen.Format(time.RFC3339Nano) + `"}`
+	if code := callAs(t, r, alice, http.MethodPost, "/api/me/notifications/read-all", body, nil); code != http.StatusNoContent {
+		t.Fatalf("read-all up_to = %d", code)
+	}
+	var count unreadCountResp
+	callAs(t, r, alice, http.MethodGet, "/api/me/notifications/unread-count", "", &count)
+	if count.UnreadCount != 1 {
+		t.Errorf("unread = %d, want 1 (the one that arrived later)", count.UnreadCount)
+	}
+	if code := callAs(t, r, alice, http.MethodPost, "/api/me/notifications/read-all", `{"up_to":"yesterday"}`, nil); code != http.StatusBadRequest {
+		t.Errorf("malformed up_to = %d, want 400", code)
+	}
+}
+
 func strPtr(s string) *string { return &s }
 
 func setSMTP(t *testing.T, pool *pgxpool.Pool, enabled bool) {