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

yarn.lock

@@ -5464,6 +5464,7 @@ __metadata:
     eslint-plugin-prettier: "npm:^5.5.5"
     prettier: "npm:^3.8.1"
     typescript: "npm:^5.9.3"
+    vitest: "npm:^0.34.1"
   languageName: unknown
   linkType: soft