Add monetization app with RevenueCat webhooks alongside Stripe.

Rename finance → monetization (keep finance_* tables via app label), add
RevenueCat webhook + ledger upserts so store IAP syncs subscriptions and
billing history like Stripe. Companion to chat_web_app#100 / #68.

llm_be/monetization/services/stripe.py

@@ -0,0 +1,139 @@
+"""Stripe Checkout, Billing Portal, and webhook dispatch."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import stripe
+from django.conf import settings
+
+from monetization.models import Invoice, SubscriptionPlan
+from monetization.services.plans import get_plan, seed_subscription_plans
+from monetization.services.webhooks import dispatch_stripe_event
+
+__all__ = [
+    "StripeNotConfiguredError",
+    "configure_stripe",
+    "resolve_checkout_plan",
+    "subscription_line_items",
+    "create_checkout_session",
+    "resolve_stripe_customer_id",
+    "create_billing_portal_session",
+    "dispatch_stripe_event",
+]
+
+
+class StripeNotConfiguredError(RuntimeError):
+    """Raised when Stripe secret key is missing."""
+
+
+def configure_stripe() -> str:
+    secret = settings.STRIPE_SECRET_KEY
+    if not secret:
+        raise StripeNotConfiguredError(
+            "STRIPE_SECRET_KEY is not configured. Set it in the environment."
+        )
+    stripe.api_key = secret
+    return secret
+
+
+def resolve_checkout_plan(plan_slug: str | None = None) -> SubscriptionPlan:
+    """Return the plan for Checkout (defaults to public Founders)."""
+    seed_subscription_plans(update_existing=False)
+    slug = (plan_slug or SubscriptionPlan.Slug.FOUNDERS).strip().lower()
+    plan = get_plan(slug)
+    if plan is None:
+        raise ValueError(f"Unknown plan: {slug}")
+    if not plan.is_selectable:
+        raise ValueError(f"Plan '{plan.slug}' is not available for checkout.")
+    return plan
+
+
+def subscription_line_items(plan: SubscriptionPlan) -> list[dict[str, Any]]:
+    """Build Checkout line_items from a SubscriptionPlan (or legacy settings)."""
+    price_id = (plan.stripe_price_id or "").strip() or (
+        settings.STRIPE_PRICE_ID if plan.slug == SubscriptionPlan.Slug.FOUNDERS else ""
+    )
+    if price_id:
+        return [{"price": price_id, "quantity": 1}]
+
+    # Founders without a plan stripe_price_id may still use legacy env amount.
+    unit_amount = plan.price_cents
+    product_name = plan.name
+    currency = plan.currency or settings.SUBSCRIPTION_PRICE_CURRENCY
+    interval = plan.interval or settings.SUBSCRIPTION_PRICE_INTERVAL
+    if plan.slug == SubscriptionPlan.Slug.FOUNDERS and not plan.stripe_price_id:
+        # Keep env overrides working for the live Founders price.
+        unit_amount = int(
+            getattr(settings, "SUBSCRIPTION_PRICE_AMOUNT_CENTS", None) or unit_amount
+        )
+        product_name = (
+            getattr(settings, "SUBSCRIPTION_PRODUCT_NAME", None) or product_name
+        )
+
+    return [
+        {
+            "price_data": {
+                "currency": currency,
+                "unit_amount": unit_amount,
+                "recurring": {"interval": interval},
+                "product_data": {"name": product_name},
+            },
+            "quantity": 1,
+        }
+    ]
+
+
+def create_checkout_session(
+    *,
+    user,
+    success_url: str | None = None,
+    cancel_url: str | None = None,
+    plan_slug: str | None = None,
+):
+    """Create a Stripe Checkout Session for a selectable subscription plan."""
+    configure_stripe()
+    plan = resolve_checkout_plan(plan_slug)
+
+    metadata = {
+        "user_id": str(user.pk),
+        "company_id": str(user.company_id) if user.company_id else "",
+        "plan_slug": plan.slug,
+    }
+    customer_email = getattr(user, "email", None) or None
+
+    session = stripe.checkout.Session.create(
+        mode="subscription",
+        line_items=subscription_line_items(plan),
+        success_url=success_url or settings.STRIPE_CHECKOUT_SUCCESS_URL,
+        cancel_url=cancel_url or settings.STRIPE_CHECKOUT_CANCEL_URL,
+        customer_email=customer_email,
+        client_reference_id=str(user.pk),
+        metadata=metadata,
+        subscription_data={"metadata": metadata},
+    )
+    return session, plan
+
+
+def resolve_stripe_customer_id(*, user) -> str | None:
+    """Return the most recent Stripe customer id stored on the user's invoices."""
+    return (
+        Invoice.objects.filter(user=user)
+        .exclude(stripe_customer_id="")
+        .order_by("-created")
+        .values_list("stripe_customer_id", flat=True)
+        .first()
+    )
+
+
+def create_billing_portal_session(
+    *,
+    customer_id: str,
+    return_url: str | None = None,
+):
+    """Create a Stripe Customer Portal session for plan/payment/cancel management."""
+    configure_stripe()
+    return stripe.billing_portal.Session.create(
+        customer=customer_id,
+        return_url=return_url or settings.STRIPE_PORTAL_RETURN_URL,
+    )