Add python-dotenv support for automatic .env loading

- Added python-dotenv to requirements.txt
- Config now automatically loads .env file if present
- Allows local development without manually exporting env vars
- Gracefully falls back if python-dotenv not installed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

requirements.txt

@@ -2,3 +2,4 @@ Flask>=2.3
 elasticsearch>=7.0.0,<9.0.0
 youtube-transcript-api>=0.6
 google-api-python-client>=2.0.0
+python-dotenv>=0.19.0