feat: add .env.example templates for development setup

- Create .env.example with database and API key placeholders
- Add backend/.env.example for backend development
- Add frontend/.env.example for Vite frontend config
- Add tests/env-example.spec.ts with 13 validation tests
- Verify .gitignore correctly ignores .env files

vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    testDir: 'tests',
+    include: ['**/*.spec.ts'],
+  },
+});