fix: send webhook milestone notifications silently (#45)

## Summary

- Adds `flags: 4096` (`MessageFlags.SUPPRESS_NOTIFICATIONS`) to the Discord webhook payload in `postMilestoneWebhook`
- Milestone announcements (prestige, transcendence, apotheosis) will now appear in the channel without triggering desktop or mobile push notifications
- Defines the magic number as a documented `suppressNotifications` constant for self-documentation
- Updates the webhook test to assert `flags: 4096` is present in the outgoing payload

Closes #41

## Test plan

- [ ] Lint passes: `pnpm lint`
- [ ] Build passes: `pnpm build`
- [ ] Tests pass with 100% coverage: `pnpm test`
- [ ] Trigger a prestige/transcendence/apotheosis in-game and verify the Discord webhook message arrives without pinging anyone

✨ This issue was created with help from Hikari~ 🌸

Reviewed-on: #45
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

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 () => {