feat(billing): add /billing/checkout-session via BillingService

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_billing_checkout.py

@@ -0,0 +1,56 @@
+import pytest
+from unittest.mock import patch, MagicMock
+from app.models.plan_billing import PlanBilling
+
+
+@pytest.mark.asyncio
+async def test_checkout_session_creates_stripe_session(
+    client, test_db, test_user, auth_headers, monkeypatch
+):
+    """End-to-end: post body → Stripe SDK called → URL returned. Stripe SDK
+    mocked; Customer + Session calls patched."""
+    from app.core.config import settings
+    monkeypatch.setattr(settings, "STRIPE_SECRET_KEY", "sk_test_dummy")
+
+    test_db.add(PlanBilling(
+        plan="pro",
+        display_name="Pro",
+        stripe_product_id="prod_test",
+        stripe_monthly_price_id="price_test_monthly",
+    ))
+    await test_db.commit()
+
+    fake_customer = MagicMock()
+    fake_customer.id = "cus_test_123"
+    fake_session = MagicMock()
+    fake_session.url = "https://checkout.stripe.com/test"
+
+    with patch("stripe.Customer.create", return_value=fake_customer) as cust_mock, \
+         patch("stripe.checkout.Session.create", return_value=fake_session) as sess_mock:
+        response = await client.post(
+            "/api/v1/billing/checkout-session",
+            json={"plan": "pro", "seats": 3, "billing_interval": "monthly"},
+            headers=auth_headers,
+        )
+
+    assert response.status_code == 200, response.json()
+    assert response.json()["url"] == "https://checkout.stripe.com/test"
+    cust_mock.assert_called_once()
+    sess_mock.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_checkout_session_unknown_plan_returns_500(
+    client, test_db, test_user, auth_headers, monkeypatch
+):
+    """No PlanBilling row → ValueError surfaces as 500 (the endpoint doesn't
+    catch business errors)."""
+    from app.core.config import settings
+    monkeypatch.setattr(settings, "STRIPE_SECRET_KEY", "sk_test_dummy")
+
+    response = await client.post(
+        "/api/v1/billing/checkout-session",
+        json={"plan": "pro", "seats": 1, "billing_interval": "monthly"},
+        headers=auth_headers,
+    )
+    assert response.status_code == 500