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

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

backend/app/schemas/billing.py

@@ -0,0 +1,12 @@
+from typing import Literal
+from pydantic import BaseModel
+
+
+class CheckoutSessionCreate(BaseModel):
+    plan: Literal["pro", "starter", "team", "enterprise"]
+    seats: int
+    billing_interval: Literal["monthly", "annual"] = "monthly"
+
+
+class CheckoutSessionResponse(BaseModel):
+    url: str