feat: implement backlog sync cooldown mechanism and update related tests

tests/routes/syncRoutes.test.ts

@@ -1,6 +1,10 @@
 import type { Request, Response } from "express";
 import { beforeEach, describe, expect, it, vi } from "vitest";
-import { createSyncRoutes } from "../../src/routes/syncRoutes";
+import {
+  clearRecentBacklogSyncs,
+  createSyncRoutes,
+  shouldSkipRecentBacklogSync,
+} from "../../src/routes/syncRoutes";
 
 const syncSelectedChannelBacklog = vi.hoisted(() => vi.fn());
 
@@ -11,6 +15,7 @@ vi.mock("../../src/moderation/backlogSync", () => ({
 describe("createSyncRoutes", () => {
   beforeEach(() => {
     syncSelectedChannelBacklog.mockReset();
+    clearRecentBacklogSyncs();
   });
 
   it("syncs the selected guild and channel from the request", async () => {
@@ -41,7 +46,52 @@ describe("createSyncRoutes", () => {
       success: true,
       channelId: "selected-channel",
       messagesSync: 3,
+      skipped: false,
     });
     expect(next).not.toHaveBeenCalled();
   });
+
+  it("skips repeated sync requests during the cooldown window", async () => {
+    expect(shouldSkipRecentBacklogSync("guild", "channel", 1000)).toBe(false);
+    expect(shouldSkipRecentBacklogSync("guild", "channel", 1001)).toBe(true);
+  });
+
+  it("allows repeated sync requests after the cooldown window", async () => {
+    expect(shouldSkipRecentBacklogSync("guild", "channel", 1000)).toBe(false);
+    expect(shouldSkipRecentBacklogSync("guild", "channel", 301001)).toBe(false);
+  });
+
+  it("does not call Discord backlog sync for repeated requests", async () => {
+    syncSelectedChannelBacklog.mockResolvedValue(3);
+    const router = createSyncRoutes({} as never);
+    const route = router.stack.find(
+      (layer) => layer.route?.path === "/backlog-sync",
+    );
+    const handler = route?.route?.stack[0]?.handle;
+
+    await handler?.(
+      {
+        body: { guildId: "selected-guild", channelId: "selected-channel" },
+      } as Request,
+      { json: vi.fn() } as unknown as Response,
+      vi.fn(),
+    );
+
+    const json = vi.fn();
+    await handler?.(
+      {
+        body: { guildId: "selected-guild", channelId: "selected-channel" },
+      } as Request,
+      { json } as unknown as Response,
+      vi.fn(),
+    );
+
+    expect(syncSelectedChannelBacklog).toHaveBeenCalledTimes(1);
+    expect(json).toHaveBeenCalledWith({
+      success: true,
+      channelId: "selected-channel",
+      messagesSync: 0,
+      skipped: true,
+    });
+  });
 });