Add environment example file for PyP100 configuration

- Created .env.example file for backend configuration
- Updated environment variable format for smart plugs
- PRINTERS variable now uses the new JSON format required by the PyP100 library

🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>

backend/.env.example

@@ -0,0 +1,6 @@
+SECRET_KEY=dev-secret-key-change-in-production
+DATABASE_PATH=instance/myp.db
+TAPO_USERNAME=your-tapo-email
+TAPO_PASSWORD=your-tapo-password
+# PRINTERS Format: {"Printer 1": {"ip": "192.168.1.100"}, "Printer 2": {"ip": "192.168.1.101"}, ...}
+PRINTERS={}