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

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

backend/app/services/billing.py

@@ -2,9 +2,14 @@
 abstraction. Account row is canonical local state; Stripe is canonical
 remote state; the webhook handler bridges the two."""
 from datetime import datetime, timezone, timedelta
+
+import stripe
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.core.config import settings
+from app.models.account import Account
+from app.models.plan_billing import PlanBilling
 from app.models.subscription import Subscription
 
 
@@ -34,3 +39,66 @@ class BillingService:
         await db.commit()
         await db.refresh(sub)
         return sub
+
+    @staticmethod
+    async def create_checkout_session(
+        db: AsyncSession,
+        account: Account,
+        plan: str,
+        seats: int,
+        billing_interval: str,
+        success_url: str,
+        cancel_url: str,
+    ) -> str:
+        """Create a Stripe Checkout Session for subscription purchase. If the
+        account currently has a trialing subscription with time remaining, that
+        trial end is preserved on the new Stripe subscription so the user
+        isn't charged early."""
+        if not settings.stripe_enabled:
+            raise RuntimeError("Stripe not configured")
+        stripe.api_key = settings.STRIPE_SECRET_KEY
+
+        plan_billing = (await db.execute(
+            select(PlanBilling).where(PlanBilling.plan == plan)
+        )).scalar_one_or_none()
+        if plan_billing is None:
+            raise ValueError(f"Unknown plan: {plan}")
+        price_id = (
+            plan_billing.stripe_monthly_price_id if billing_interval == "monthly"
+            else plan_billing.stripe_annual_price_id
+        )
+        if price_id is None:
+            raise RuntimeError(
+                f"Plan '{plan}' has no Stripe price for {billing_interval}"
+            )
+
+        if account.stripe_customer_id is None:
+            customer = stripe.Customer.create(
+                email=None,
+                metadata={"account_id": str(account.id)},
+            )
+            account.stripe_customer_id = customer.id
+            await db.commit()
+
+        sub = (await db.execute(
+            select(Subscription).where(Subscription.account_id == account.id)
+        )).scalar_one_or_none()
+        subscription_data = {}
+        if (
+            sub
+            and sub.status == "trialing"
+            and sub.current_period_end
+            and sub.current_period_end > datetime.now(timezone.utc)
+        ):
+            subscription_data["trial_end"] = int(sub.current_period_end.timestamp())
+
+        session = stripe.checkout.Session.create(
+            customer=account.stripe_customer_id,
+            line_items=[{"price": price_id, "quantity": seats}],
+            mode="subscription",
+            subscription_data=subscription_data or None,
+            success_url=success_url,
+            cancel_url=cancel_url,
+            allow_promotion_codes=False,
+        )
+        return session.url