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

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

backend/app/api/endpoints/billing.py

@@ -0,0 +1,36 @@
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.api.deps import get_current_active_user
+from app.core.admin_database import get_admin_db
+from app.core.config import settings
+from app.models.account import Account
+from app.models.user import User
+from app.schemas.billing import CheckoutSessionCreate, CheckoutSessionResponse
+from app.services.billing import BillingService
+
+router = APIRouter(prefix="/billing", tags=["billing"])
+
+
+@router.post("/checkout-session", response_model=CheckoutSessionResponse)
+async def create_checkout_session(
+    payload: CheckoutSessionCreate,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> CheckoutSessionResponse:
+    account = (await db.execute(
+        select(Account).where(Account.id == current_user.account_id)
+    )).scalar_one()
+    url = await BillingService.create_checkout_session(
+        db=db,
+        account=account,
+        plan=payload.plan,
+        seats=payload.seats,
+        billing_interval=payload.billing_interval,
+        success_url=f"{settings.FRONTEND_URL}/account/billing?success=1",
+        cancel_url=f"{settings.FRONTEND_URL}/account/billing/select-plan",
+    )
+    return CheckoutSessionResponse(url=url)

backend/app/api/router.py

@@ -23,6 +23,7 @@ from app.api.endpoints import (
     analytics,
     assistant_chat,
     auth,
+    billing,
     beta_feedback,
     beta_signup,
     branding,
@@ -81,6 +82,7 @@ api_router = APIRouter()
 # in Phase 1. This will need revisiting in Phase 2 when `users` gets RLS.
 # ---------------------------------------------------------------------------
 api_router.include_router(auth.router)
+api_router.include_router(billing.router)     # Reachable when subscription locked
 api_router.include_router(shared.router)       # Public share links (no auth)
 api_router.include_router(shares.public_router)  # Public session share links (optional auth)
 api_router.include_router(beta_signup.router)