fix : validate SECRET_KEY is set and long enough at startup

[tmdeveloper007]

Summary of What Needs to be Done:
The SECRET_KEY setting in backend/app/core/config.py accepts empty strings without raising a validation error. If the .env file contains SECRET_KEY= (empty value), pydantic-settings silently accepts it and the app starts with an insecure key, allowing trivial JWT forgery.

Changes that Need to be Made:

• backend/app/core/config.py: Add a pydantic @field_validator on SECRET_KEY that raises ValueError if the value is empty or shorter than 32 characters. Provide a clear error message: generate one with: openssl rand -hex 32

Impact that it would Provide:

• Fails fast and loudly if SECRET_KEY is misconfigured
• Prevents silent insecurity in production deployments
• Aligns with the principle of secure defaults

Note: Please assign this issue to the tmdeveloper007 account.