Add a Mistral engine

[r10d1nsec]

Add Mistral as an API engine (OpenAI-compatible, base URL https://api.mistral.ai/v1).

Where: backend/services/engines/ (new mistral.py), register in __init__.py build_engine(), add a test in tests/test_engines.py, expose it in frontend/src/screens/Engines.tsx.

Guide: there's a full step-by-step "add your first engine in ~20 minutes" walkthrough in CONTRIBUTING.md using Mistral as the example. Great first PR.