feat: add retry(10) on all network requests

- New src/retry.py with configurable retry decorator
- Applied @retry() to fetch_unseen_emails, mark_as_seen,
  summarize_email, and send_message
- No backoff delay between retries

src/tg_bot.py

@@ -1,7 +1,9 @@
 import requests
 from src.config import TelegramConfig
+from src.retry import retry
 
 
+@retry()
 def send_message(tg_cfg: TelegramConfig, text: str):
     url = f"https://api.telegram.org/bot{tg_cfg.bot_token}/sendMessage"
     payload = {