refactor: full async, rate limit backoff, graceful shutdown, health endpoint

- Switch from requests to httpx.AsyncClient (full async HTTP)
- Add _request() with exponential backoff on 429/401
- Cache followees with 5-min TTL to reduce API calls
- Remove dead code (_get_all_stories, instagram_password, session_file)
- Graceful shutdown via asyncio.Event + SIGTERM/SIGINT handlers
- Add /health command with connection status
- Stream media downloads with httpx + aiofiles
- Track story expiring_at for smart cleanup
- Sanitize .env.example (remove real tokens)

README.md

@@ -74,6 +74,7 @@ docker compose up -d
 | `/start` | Информация о боте |
 | `/check` | Проверить обновления сейчас |
 | `/status` | Статистика базы |
+| `/health` | Статус подключений |
 | `/stop` | Остановить мониторинг |
 
 ## API