feat: add Telegram configuration to settings (issue #33)

Add Telegram notification configuration:
- src/config.py: Add TELEGRAM_BOT_TOKEN, TELEGRAM_CHAT_ID, TELEGRAM_ENABLED
- .env.example: Add Telegram section with setup instructions

Fields added after S3_REGION (line 55).
Follows existing optional API pattern (NEWS_API_KEY, etc.).
No breaking changes to existing settings.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/config.py

@@ -54,6 +54,11 @@ class Settings(BaseSettings):
     S3_BUCKET_NAME: str | None = None
     S3_REGION: str = "us-east-1"
 
+    # Telegram Notifications (optional)
+    TELEGRAM_BOT_TOKEN: str | None = None
+    TELEGRAM_CHAT_ID: str | None = None
+    TELEGRAM_ENABLED: bool = True
+
     model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}
 
     @property