chore: remove sientia models module and relocate date format validation

- Deleted the sientia/models.py file, which contained the LinearRegressionModel and related functionality.
- Moved the frontend date format validation logic to train_model_params.py, ensuring a single source of truth for date formats.

model_manager/utils/models/train_model_params.py

@@ -3,7 +3,25 @@ from typing import Any
 
 from jsonschema import Draft202012Validator, ValidationError  # type: ignore[import-untyped]
 
-from model_manager.sientia.models import validate_frontend_date_format
+# Allowed frontend date formats and their strftime equivalents (single source of truth)
+FRONTEND_DATE_FORMAT_TO_STRFTIME = {
+    'dd/MM/yyyy HH:mm:ss': '%d/%m/%Y %H:%M:%S',
+    'MM/dd/yyyy HH:mm:ss': '%m/%d/%Y %H:%M:%S',
+    'yyyy/MM/dd HH:mm:ss': '%Y/%m/%d %H:%M:%S',
+    'dd-MM-yyyy HH:mm:ss': '%d-%m-%Y %H:%M:%S',
+    'MM-dd-yyyy HH:mm:ss': '%m-%d-%Y %H:%M:%S',
+    'yyyy-MM-dd HH:mm:ss': '%Y-%m-%d %H:%M:%S',
+}
+ALLOWED_FRONTEND_DATE_FORMATS = frozenset(FRONTEND_DATE_FORMAT_TO_STRFTIME.keys())
+
+
+def validate_frontend_date_format(fmt: str | None) -> None:
+    """Raise ValueError if fmt is set and not one of the allowed frontend date formats."""
+    if not fmt or not fmt.strip():
+        return
+    if fmt not in ALLOWED_FRONTEND_DATE_FORMATS:
+        allowed = ', '.join(sorted(ALLOWED_FRONTEND_DATE_FORMATS))
+        raise ValueError(f'Invalid date_format "{fmt}". Allowed formats: {allowed}')
 
 # Model name constants
 MODEL_LINEAR_REGRESSION = 'Linear Regression'