fix(webhook): send milestone notifications with SUPPRESS_NOTIFICATIONS flag

Adds flags: 4096 (MessageFlags.SUPPRESS_NOTIFICATIONS) to the Discord
webhook payload so milestone announcements appear in the channel without
triggering desktop or mobile push notifications. Closes #41.

apps/api/test/services/webhook.spec.ts

@@ -97,9 +97,10 @@ describe("webhook service", () => {
       await postMilestoneWebhook("user123", "prestige", counts);
       const [url, options] = mockFetch.mock.calls[0] as [string, RequestInit];
       expect(url).toBe("https://discord.com/webhook/abc");
-      const body = JSON.parse(options.body as string) as { content: string };
+      const body = JSON.parse(options.body as string) as { content: string; flags: number };
       expect(body.content).toContain("<@user123>");
       expect(body.content).toContain("prestiged");
+      expect(body.flags).toBe(4096);
     });
 
     it("posts transcendence message correctly", async () => {