feat: add cursor-based message queries

tests/moderation/messageStoreQueries.test.ts

@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vitest";
+import { decodeCursor, encodeCursor } from "../../src/moderation/messageStore";
+
+describe("message cursor helpers", () => {
+  it("round-trips created_at and id", () => {
+    const cursor = encodeCursor({ created_at: 1710000000000, id: "abc" });
+    expect(decodeCursor(cursor)).toEqual({ created_at: 1710000000000, id: "abc" });
+  });
+
+  it("returns null for invalid cursor", () => {
+    expect(decodeCursor("not-base64-json")).toBeNull();
+  });
+});