Implement HMAC-SHA256 signature verification for model files to mitigate joblib/pickle deserialization risks.
Key changes:
- Added `MODEL_SIGNING_KEY` to `TradingConfig`.
- Created `src/utils/security.py` for cryptographic integrity checks.
- Hardened `RegimeDetector` to sign models on save and verify on load.
- Updated `ConfigValidator` to ensure signing keys are configured.
- Added `.pip-audit.ignore` and updated CI workflows to manage known upstream vulnerabilities.
- Comprehensive test coverage in `tests/test_model_integrity.py`.
Security Impact: Prevents arbitrary code execution via malicious model files by requiring a valid HMAC signature derived from a secret key.
Co-authored-by: xnessom <255288032+xnessom@users.noreply.github.com>
