Add finance app with Stripe Checkout subscriptions (#21)

Introduce Invoice/Payment ledger models, $10/mo settings-backed pricing,
hosted Checkout + signed webhooks, Django admin, and authenticated list APIs.

llm_be/finance/services/stripe_service.py

@@ -0,0 +1,71 @@
+"""Stripe Checkout session helpers."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import stripe
+from django.conf import settings
+
+
+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 subscription_line_items() -> list[dict[str, Any]]:
+    """Build Checkout line_items from settings-backed subscription pricing."""
+    price_id = settings.STRIPE_PRICE_ID
+    if price_id:
+        return [{"price": price_id, "quantity": 1}]
+
+    return [
+        {
+            "price_data": {
+                "currency": settings.SUBSCRIPTION_PRICE_CURRENCY,
+                "unit_amount": settings.SUBSCRIPTION_PRICE_AMOUNT_CENTS,
+                "recurring": {"interval": settings.SUBSCRIPTION_PRICE_INTERVAL},
+                "product_data": {
+                    "name": settings.SUBSCRIPTION_PRODUCT_NAME,
+                },
+            },
+            "quantity": 1,
+        }
+    ]
+
+
+def create_checkout_session(
+    *,
+    user,
+    success_url: str | None = None,
+    cancel_url: str | None = None,
+):
+    """Create a Stripe Checkout Session for the subscription plan."""
+    configure_stripe()
+
+    metadata = {
+        "user_id": str(user.pk),
+        "company_id": str(user.company_id) if user.company_id else "",
+    }
+    customer_email = getattr(user, "email", None) or None
+
+    session = stripe.checkout.Session.create(
+        mode="subscription",
+        line_items=subscription_line_items(),
+        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