Add secret_key, CSP headers, fix debug mode

[Akshita-2307]

Summary

• Add import os and dynamic secret_key via FLASK_SECRET_KEY env var or os.urandom(24).hex()
• Add Content-Security-Policy header
• Make debug mode configurable via FLASK_DEBUG env var (fixes [Bug]: Missing secret_key, hardcoded debug=True and incomplete security headers in app.py #646)

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[Akshita-2307]

Program: GSSoC