feat(billing): add BillingService.start_trial; wire into /auth/register

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

backend/tests/test_billing_service.py

@@ -0,0 +1,59 @@
+import uuid
+import pytest
+from datetime import datetime, timezone
+from sqlalchemy import select, delete
+from app.models.subscription import Subscription
+from app.services.billing import BillingService
+
+
+@pytest.mark.asyncio
+async def test_start_trial_creates_trialing_pro_subscription(test_db):
+    """Direct service test — bypasses register, creates account inline."""
+    from app.models.account import Account
+    account = Account(name="DirectTest", display_code="DIRECT01")
+    test_db.add(account)
+    await test_db.flush()
+
+    sub = await BillingService.start_trial(test_db, account.id)
+    assert sub.plan == "pro"
+    assert sub.status == "trialing"
+    assert sub.current_period_end is not None
+    assert sub.current_period_end > datetime.now(timezone.utc)
+
+
+@pytest.mark.asyncio
+async def test_start_trial_is_idempotent(test_db):
+    from app.models.account import Account
+    account = Account(name="Idempo", display_code="IDEMPO01")
+    test_db.add(account)
+    await test_db.flush()
+
+    sub1 = await BillingService.start_trial(test_db, account.id)
+    sub2 = await BillingService.start_trial(test_db, account.id)
+    assert sub1.id == sub2.id
+
+    rows = (await test_db.execute(
+        select(Subscription).where(Subscription.account_id == account.id)
+    )).scalars().all()
+    assert len(rows) == 1
+
+
+@pytest.mark.asyncio
+async def test_register_creates_trial_subscription(client, test_db):
+    """Registering a brand-new shop (no invite code) yields a Pro/trialing sub."""
+    response = await client.post("/api/v1/auth/register", json={
+        "email": "newshop@example.com",
+        "password": "Verystrong1Pwd",
+        "name": "New Shop",
+    })
+    assert response.status_code in (200, 201), response.json()
+
+    body = response.json()
+    account_id = uuid.UUID(body["account_id"])
+
+    sub = (await test_db.execute(
+        select(Subscription).where(Subscription.account_id == account_id)
+    )).scalar_one()
+    assert sub.plan == "pro"
+    assert sub.status == "trialing"
+    assert sub.current_period_end is not None