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>

.env.example

@@ -26,3 +26,10 @@ MODE=paper
 # NEWS_API_KEY=your_news_api_key_here
 # NEWS_API_PROVIDER=alphavantage
 # MARKET_DATA_API_KEY=your_market_data_key_here
+
+# Telegram Notifications (optional)
+# Get bot token from @BotFather on Telegram
+# Get chat ID from @userinfobot or your chat
+# TELEGRAM_BOT_TOKEN=1234567890:ABCdefGHIjklMNOpqrsTUVwxyz
+# TELEGRAM_CHAT_ID=123456789
+# TELEGRAM_ENABLED=true

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