Add Jest unit test infrastructure and initial test suite

Sets up Jest + ts-jest for the backend and adds 47 tests across 5
suites covering crypto utilities, OUI vendor lookup, auth middleware,
AlertService dispatch/cooldown logic, and DeviceCollector uptime parsing.

backend/jest.config.js

@@ -0,0 +1,12 @@
+/** @type {import('jest').Config} */
+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+  testMatch: ['**/__tests__/**/*.test.ts'],
+  collectCoverageFrom: [
+    'src/**/*.ts',
+    '!src/db/migrations/**',
+    '!src/index.ts',
+  ],
+  coverageReporters: ['text', 'lcov'],
+};