Fix test conftest missing 4 model imports causing slot_preset_mappings table error

  The test conftest.py model import list was out of sync with database.py,
  missing slot_preset, project_bom, spool_k_profile, and spoolbuddy_device.
  Base.metadata.create_all() never created those tables in the test DB.

backend/tests/conftest.py

@@ -81,12 +81,16 @@ async def test_engine():
         print_queue,
         printer,
         project,
+        project_bom,
         settings,
+        slot_preset,
         smart_plug,
         spool,
         spool_assignment,
         spool_catalog,
+        spool_k_profile,
         spool_usage_history,
+        spoolbuddy_device,
         user,
         user_email_pref,
         virtual_printer,