Add unit tests for the grid trading strategy

[sanmaxdev]

Background

The core trading logic in backend/app/strategy/ (grid levels, indicators, state machine, liquidation) currently has no automated test coverage. Solid tests here give contributors confidence to change the strategy safely.

Tasks

• Add pytest tests for backend/app/strategy/grid.py (grid level + TP price math)
• Add tests for backend/app/strategy/indicators.py (signal evaluation)
• Add tests for backend/app/strategy/state_machine.py
• Use fixtures for sample candle/price data (no live Binance calls)

Acceptance criteria

• pytest -v passes locally and in CI
• New code paths are covered with clear, isolated unit tests

New to the project? See CONTRIBUTING.md. Happy to help scope this further.