test(5.1): add integration and unit tests for quick quantity adjustment

- Vitest hook tests for useQuantityAdjustment (optimistic updates, debouncing)
- Pytest backend tests for PATCH /items/{itemId} endpoint
- Tests cover success, failure, validation, and audit logging scenarios

backend/tests/test_quantity_patch.py

@@ -0,0 +1,165 @@
+"""
+Test PATCH /items/{item_id} endpoint for quantity adjustment
+Ensures audit logging and validation work correctly
+"""
+
+import pytest
+import json
+from fastapi.testclient import TestClient
+from sqlalchemy.orm import Session
+from ..models import Item, Category, AuditLog
+from ..schemas import Item as ItemSchema
+from ..database import Base, engine
+
+
+@pytest.fixture
+def test_item(db: Session):
+    """Create a test item"""
+    category = Category(name="Test Category")
+    db.add(category)
+    db.commit()
+
+    item = Item(
+        name="Test Item",
+        category="Test Category",
+        type="Test Type",
+        quantity=10,
+        barcode="TEST123",
+        part_number="PN-TEST-001"
+    )
+    db.add(item)
+    db.commit()
+    db.refresh(item)
+    return item
+
+
+def test_patch_quantity_success(client: TestClient, test_item: Item, db: Session, auth_token: str):
+    """Test successful quantity update with audit logging"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 25},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+    data = response.json()
+    assert data["quantity"] == 25
+    assert data["id"] == test_item.id
+
+    # Verify audit log was created
+    audit_entries = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).all()
+
+    assert len(audit_entries) == 1
+    audit = audit_entries[0]
+    assert audit.quantity_change == 15  # 25 - 10
+    assert audit.target_item_name == "Test Item"
+    assert audit.target_item_pn == "PN-TEST-001"
+    assert "10 → 25" in audit.details
+
+
+def test_patch_quantity_to_zero(client: TestClient, test_item: Item, db: Session, auth_token: str):
+    """Test setting quantity to zero"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 0},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+    data = response.json()
+    assert data["quantity"] == 0
+
+    # Verify audit log has correct delta
+    audit = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).first()
+
+    assert audit.quantity_change == -10  # 0 - 10
+
+
+def test_patch_quantity_negative_fails(client: TestClient, test_item: Item, auth_token: str):
+    """Test that negative quantity is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": -5},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "non-negative" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_missing_field(client: TestClient, test_item: Item, auth_token: str):
+    """Test that missing quantity field is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "quantity" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_invalid_type(client: TestClient, test_item: Item, auth_token: str):
+    """Test that non-integer quantity is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": "not a number"},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "integer" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_not_found(client: TestClient, auth_token: str):
+    """Test that updating non-existent item returns 404"""
+    response = client.patch(
+        "/items/99999",
+        json={"quantity": 10},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 404
+    assert "not found" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_no_auth(client: TestClient, test_item: Item):
+    """Test that unauthenticated request is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 25}
+    )
+
+    assert response.status_code == 401
+
+
+def test_patch_quantity_audit_fields(client: TestClient, test_item: Item, db: Session, auth_token: str, current_user_id: str):
+    """Test that audit log contains all required fields"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 30},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+
+    audit = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).first()
+
+    assert audit is not None
+    assert audit.user_id == current_user_id
+    assert audit.action == "UPDATE_QUANTITY"
+    assert audit.target_item_id == test_item.id
+    assert audit.target_item_name == "Test Item"
+    assert audit.target_item_pn == "PN-TEST-001"
+    assert audit.target_item_barcode == "TEST123"
+    assert audit.quantity_change == 20  # 30 - 10
+    assert "→" in audit.details  # Direction indicator in details