fix(deps): declare psutil in requirements.txt — green API Documentation CI

The API Documentation job (and any env without locust) failed with
`ModuleNotFoundError: No module named 'psutil'` when importing the app:
psutil is imported by src/api/routers/health.py, services/metrics.py,
commands/status.py, and tasks/monitoring.py, but was never declared as a
dependency — it only happened to be present where locust (Performance
Tests) pulled it in transitively. Declare it explicitly (psutil>=5.9.0).

Verified locally: `from src.api.main import app; app.openapi()` (the exact
docs-job operation) now succeeds.

requirements.txt

@@ -36,3 +36,4 @@ scikit-learn>=1.2.0
 
 # Monitoring dependencies
 prometheus-client>=0.16.0
+psutil>=5.9.0  # system metrics — imported by health.py / metrics.py / status.py / monitoring.py