feat(models): add Station and StationRecipeAssignment models

TDD: test written first, confirmed failing with ModuleNotFoundError,
then model implemented; all 3 new tests pass. conftest updated to
import new models so Base.metadata.create_all picks up the tables.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/tests/conftest.py

@@ -39,6 +39,7 @@ from models.task import RecipeTask, RecipeSubtask
 from models.measurement import Measurement
 from models.access_log import AccessLog
 from models.setting import SystemSetting, RecipeVersionAudit
+from models.station import Station, StationRecipeAssignment
 from services.auth_service import hash_password, generate_api_key
 
 # ---------------------------------------------------------------------------