refactor: move platform API database queries to service layer

- Create platform_signup_service.py for signup flow operations
- Create platform_pricing_service.py for pricing data operations
- Refactor signup.py, pricing.py, letzshop_vendors.py to use services
- Add # public markers to all platform endpoints
- Update tests for correct mock paths and status codes

Fixes architecture validation errors (API-002, API-003, SVC-006).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/api/v1/platform/test_signup.py

@@ -17,7 +17,7 @@ from models.database.vendor import Vendor
 @pytest.fixture
 def mock_stripe_service():
     """Mock the Stripe service for tests."""
-    with patch("app.api.v1.platform.signup.stripe_service") as mock:
+    with patch("app.services.platform_signup_service.stripe_service") as mock:
         mock.create_customer.return_value = "cus_test_123"
         mock.create_setup_intent.return_value = MagicMock(
             id="seti_test_123",
@@ -147,7 +147,7 @@ class TestSignupStartAPI:
             json={"tier_code": "invalid_tier", "is_annual": False},
         )
 
-        assert response.status_code == 400
+        assert response.status_code == 422  # ValidationException
         data = response.json()
         assert "invalid tier" in data["message"].lower()
 
@@ -226,7 +226,7 @@ class TestClaimVendorAPI:
             },
         )
 
-        assert response.status_code == 400
+        assert response.status_code == 409  # ConflictException
         data = response.json()
         assert "already claimed" in data["message"].lower()
 
@@ -309,7 +309,7 @@ class TestCreateAccountAPI:
             },
         )
 
-        assert response.status_code == 400
+        assert response.status_code == 409  # ConflictException
         data = response.json()
         assert "already exists" in data["message"].lower()
 
@@ -414,7 +414,7 @@ class TestSetupPaymentAPI:
             json={"session_id": signup_session},
         )
 
-        assert response.status_code == 400
+        assert response.status_code == 422  # ValidationException
         data = response.json()
         assert "account not created" in data["message"].lower()
 
@@ -514,7 +514,7 @@ class TestCompleteSignupAPI:
             },
         )
 
-        assert response.status_code == 400
+        assert response.status_code == 422  # ValidationException
         data = response.json()
         assert "not completed" in data["message"].lower()