[ENHANCEMENT] Secure chatbot backend

[Aditya8369]

Secure chatbot backend

Step 1: Understand current chatbot implementation

• Read chatbot.py to identify security gaps (input validation, rate limiting, unsafe formatting).

Step 2: Implement fixes

• Update chatbot.py:
  • Keep API key from GEMINI_API_KEY env var (no hardcoding)
  • Add request validation (type + max length)
  • Add rate limiting per IP (in-memory sliding window)
  • Add basic abuse/prompt-injection guardrails (keyword heuristics + educational scope)
  • Make output escaping safe (remove unsafe HTML tag conversion)
  • Improve robustness when reading Gemini response parts

closes #46

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