feat: add SQLite persistence and REST API for temporary whitelisting

- Migrate IP-based temporary whitelisting from file to SQLite storage
- Add REST API endpoints for managing temporary and permanent whitelists
- Create `.env.example` with required environment variables
- Document API endpoints in README.md and docs/api.md
- Add new dependency `modernc.org/sqlite` for SQLite support
- Update deployment and security documentation

.env.example

@@ -0,0 +1,5 @@
+AUTH_GATE_PORT
+AUTH_GATE_API_TOKEN
+DATA_DIR
+CLEANUP_INTERVAL
+AUTH_GATE_DB_PATH