feat: add configuration for keeping deleted pastes and update trusted hosts format

- Introduce `APP_KEEP_DELETED_PASTES_TIME_HOURS` to control how long deleted pastes are retained
- Rename `TRUSTED_HOSTS` to `APP_TRUSTED_HOSTS` for consistency
- Update `.env.example` with new configuration option and corrected variable name

Author: Pdzly

.env.example

@@ -18,7 +18,8 @@ APP_CORS_DOMAINS='["http://localhost:3000"]'
 APP_CACHE_TTL=300
 APP_CACHE_SIZE_LIMIT=1000
 APP_MIN_STORAGE_MB=1024
-TRUSTED_HOSTS='["127.0.0.1", 'devbin_frontend']'
+APP_TRUSTED_HOSTS='["127.0.0.1", 'devbin_frontend']'
+APP_KEEP_DELETED_PASTES_TIME_HOURS=336 # 2 Weeks
 
 # Debug / Development settings
 APP_SQLALCHEMY_ECHO=false