Fix: Prevent abuse of verify-payment endpoint by adding rate-limiting & idempotency (#205) | Fix: persist selected model in localStorage (#204)

[mitgajera]

Description

• Initialize the model state from localStorage if available.
• Save the selected model to localStorage whenever it changes.
  Local preference of LLM should be stored #204

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• Added an idempotency check so each transaction is only processed once.
• Implemented rate limiting on verify-payment to block spamming attempts.
• Improved logging for duplicate or suspicious requests.
• Added tests to cover duplicate handling and rate limiting.
  User can get a lot of credits by spamming the verify-payment endpoint #205