test: add comprehensive test suite for server

- Add load testing with Artillery (smoke, standard, peak, stress)
- Add functional tests: voting, lottery, admin (18 tests)
- Add fault tolerance tests (14 tests)
- Add network tests for WebSocket (7 tests)
- Add compatibility tests (15 tests)
- Add security tests: anti-fraud, auth, data integrity (10 tests)
- Fix optional callback handling in socket handlers
- Total: 64 test cases, all passing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/server/vitest.config.ts

@@ -0,0 +1,15 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html'],
+      include: ['src/services/**/*.ts'],
+    },
+    testTimeout: 10000,
+  },
+});