fix(models): align Station unique constraint + extend tests

- Station.code: usa UniqueConstraint("code", name="uq_stations_code")
  esplicito in __table_args__ invece di unique=True sulla colonna,
  per allineamento con la migration 002 ed evitare drift Alembic.
- Aggiunge test test_duplicate_assignment_is_rejected per coprire
  il vincolo uq_station_recipe (regola business centrale del modello).
- Sposta import IntegrityError a module-level per consistenza.

Feedback da code-reviewer su Task 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/tests/test_station_model.py

@@ -1,6 +1,7 @@
 """Test the Station and StationRecipeAssignment ORM models."""
 import pytest
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from models.station import Station, StationRecipeAssignment
@@ -24,7 +25,6 @@ async def test_create_station(db_session: AsyncSession):
 
 
 async def test_station_code_is_unique(db_session: AsyncSession):
-    from sqlalchemy.exc import IntegrityError
     admin = await _create_user(db_session, username="admin2", is_admin=True)
     db_session.add(Station(code="ST-DUP", name="A", created_by=admin.id))
     await db_session.flush()
@@ -53,3 +53,21 @@ async def test_assign_recipe_to_station(db_session: AsyncSession):
     assignments = result.scalars().all()
     assert len(assignments) == 1
     assert assignments[0].recipe_id == recipe.id
+
+
+async def test_duplicate_assignment_is_rejected(db_session: AsyncSession):
+    admin = await _create_user(db_session, username="admin4", is_admin=True)
+    station = Station(code="ST-003", name="Linea 3", created_by=admin.id)
+    db_session.add(station)
+    await db_session.flush()
+    recipe = await create_test_recipe(db_session, user_id=admin.id, code="REC-Y")
+    db_session.add(StationRecipeAssignment(
+        station_id=station.id, recipe_id=recipe.id, assigned_by=admin.id,
+    ))
+    await db_session.flush()
+    db_session.add(StationRecipeAssignment(
+        station_id=station.id, recipe_id=recipe.id, assigned_by=admin.id,
+    ))
+    with pytest.raises(IntegrityError):
+        await db_session.flush()
+    await db_session.rollback()