fix(config): call load_dotenv() in config.py and add gemini-flash-lite-latest alias

[anmolsharma152]

Problem

1. config.py evaluates os.getenv("DEFAULT_MODEL", ...) at import time, but load_dotenv() was only called later in prompt.py. As a result, .env settings were ignored when importing config.py directly.
2. New Gemini API keys hit 404 Not Found errors on deprecated snapshot model names (gemini-2.5-flash).

Solution

1. Add from dotenv import load_dotenv and load_dotenv() at the top of config.py.
2. Add gemini-flash-lite-latest model alias to providers.json.

Testing

• Verified .env variables load automatically on config.py import.
• Successfully ran score.py using DEFAULT_MODEL=gemini-flash-lite-latest.