Improve Gemini prompt to detect investment fraud patterns

The current Gemini prompt handles common scam types well but can miss investment fraud patterns common in India such as:

• Fake stock tips via WhatsApp
• Ponzi scheme recruitment messages
• Fake trading app referrals promising high returns

Update the prompt in gemini_service.py to explicitly include these patterns in the scam type detection list and add relevant examples to the system instructions.

Files to modify: backend/services/gemini_service.py