fix(billing): complete billing module — fix tier change, platform support, merchant portal

- Fix admin tier change: resolve tier_code→tier_id in update_subscription(),
  delegate to billing_service.change_tier() for Stripe-connected subs
- Add platform support to admin tiers page: platform column, filter dropdown,
  platform selector in create/edit modal, platform_name in tier API response
- Filter used platforms in create subscription modal on merchant detail page
- Enrich merchant portal API responses with tier code, tier_name, platform_name
- Add eager-load of platform relationship in get_merchant_subscription()
- Remove stale store_name/store_code references from merchant templates
- Add merchant tier change endpoint (POST /change-tier) and tier selector UI
  replacing broken requestUpgrade() button
- Fix subscription detail link to use platform_id instead of sub.id

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

app/modules/tenancy/tests/unit/test_user_model.py

@@ -0,0 +1,105 @@
+# tests/unit/models/database/test_user.py
+"""Unit tests for User database model."""
+
+import pytest
+from sqlalchemy.exc import IntegrityError
+
+from app.modules.tenancy.models import User
+
+
+@pytest.mark.unit
+@pytest.mark.database
+class TestUserModel:
+    """Test User model."""
+
+    def test_user_creation(self, db):
+        """Test User model creation and relationships."""
+        user = User(
+            email="db_test@example.com",
+            username="dbtest",
+            hashed_password="hashed_password_123",
+            role="user",
+            is_active=True,
+        )
+
+        db.add(user)
+        db.commit()
+        db.refresh(user)
+
+        assert user.id is not None
+        assert user.email == "db_test@example.com"
+        assert user.username == "dbtest"
+        assert user.role == "user"
+        assert user.is_active is True
+        assert user.created_at is not None
+        assert user.updated_at is not None
+
+    def test_user_email_uniqueness(self, db):
+        """Test email unique constraint."""
+        user1 = User(
+            email="unique@example.com",
+            username="user1",
+            hashed_password="hash1",
+        )
+        db.add(user1)
+        db.commit()
+
+        # Duplicate email should raise error
+        with pytest.raises(IntegrityError):
+            user2 = User(
+                email="unique@example.com",
+                username="user2",
+                hashed_password="hash2",
+            )
+            db.add(user2)
+            db.commit()
+
+    def test_user_username_uniqueness(self, db):
+        """Test username unique constraint."""
+        user1 = User(
+            email="user1@example.com",
+            username="sameusername",
+            hashed_password="hash1",
+        )
+        db.add(user1)
+        db.commit()
+
+        # Duplicate username should raise error
+        with pytest.raises(IntegrityError):
+            user2 = User(
+                email="user2@example.com",
+                username="sameusername",
+                hashed_password="hash2",
+            )
+            db.add(user2)
+            db.commit()
+
+    def test_user_default_values(self, db):
+        """Test User model default values."""
+        user = User(
+            email="defaults@example.com",
+            username="defaultuser",
+            hashed_password="hash",
+        )
+        db.add(user)
+        db.commit()
+        db.refresh(user)
+
+        assert user.is_active is True  # Default
+        assert user.role == "store"  # Default (UserRole.STORE)
+
+    def test_user_optional_fields(self, db):
+        """Test User model with optional fields."""
+        user = User(
+            email="optional@example.com",
+            username="optionaluser",
+            hashed_password="hash",
+            first_name="John",
+            last_name="Doe",
+        )
+        db.add(user)
+        db.commit()
+        db.refresh(user)
+
+        assert user.first_name == "John"
+        assert user.last_name == "Doe"