fix: add periodic flush and graceful shutdown for server-side telemetry

The TelemetryClient only flushed at 50 events, so the server silently
lost all queued telemetry on restart. Add startPeriodicFlush/stop methods
to TelemetryClient, wire up 60s periodic flush in server initTelemetry,
and flush on SIGTERM/SIGINT before exit.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/__tests__/telemetry-client-flush.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
+import { TelemetryClient } from "../../../packages/shared/src/telemetry/client.js";
+import type { TelemetryConfig, TelemetryState } from "../../../packages/shared/src/telemetry/types.js";
+
+function makeClient(config?: Partial<TelemetryConfig>) {
+  const merged: TelemetryConfig = { enabled: true, endpoint: "http://localhost:9999/ingest", ...config };
+  const state: TelemetryState = {
+    installId: "test-install",
+    salt: "test-salt",
+    createdAt: "2026-01-01T00:00:00Z",
+    firstSeenVersion: "0.0.0",
+  };
+  return new TelemetryClient(merged, () => state, "0.0.0-test");
+}
+
+describe("TelemetryClient periodic flush", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.stubGlobal("fetch", vi.fn().mockResolvedValue({ ok: true }));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    vi.restoreAllMocks();
+  });
+
+  it("flushes queued events on interval", async () => {
+    const client = makeClient();
+    client.startPeriodicFlush(1000);
+
+    client.track("install.started");
+    expect(fetch).not.toHaveBeenCalled();
+
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(fetch).toHaveBeenCalledTimes(1);
+
+    // Second tick with no new events — no additional call
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(fetch).toHaveBeenCalledTimes(1);
+
+    // New event gets flushed on next tick
+    client.track("install.started");
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(fetch).toHaveBeenCalledTimes(2);
+
+    client.stop();
+  });
+
+  it("stop() prevents further flushes", async () => {
+    const client = makeClient();
+    client.startPeriodicFlush(1000);
+
+    client.track("install.started");
+    client.stop();
+
+    await vi.advanceTimersByTimeAsync(2000);
+    expect(fetch).not.toHaveBeenCalled();
+  });
+
+  it("startPeriodicFlush is idempotent", () => {
+    const client = makeClient();
+    client.startPeriodicFlush(1000);
+    client.startPeriodicFlush(1000); // should not throw or double-fire
+    client.stop();
+  });
+});