Multi-plan subscriptions, quotas, and token usage APIs (#16 #17 #36) (#37)

## Summary
Implements [#16](#16), [#17](#17), and [#36](#36) in one backend PR.

- **#36 Multi-plan catalog**: Founders ($10, public), Standard ($15), Pro ($40), Business ($99), Backer ($0). Future tiers seeded but hidden/`is_selectable=false`. Backer email whitelist auto-assigns Founders-level access with no checkout.
- **#36 Feature + prompt gating**: plan feature flags (text vs image); rolling **6h** prompt windows (100 / 200 / 300 / 300 / 300). Enforced in both chat consumers when `ENFORCE_SUBSCRIPTION_GATES=true`.
- **#17 Token-period quotas**: optional `monthly_token_quota` on plans + per-user override; calendar-month aggregation from `PromptMetric`; warn/block when reported token totals exceed cap. Null provider usage never fabricated as 0; tracked via `turns_missing_token_usage`.
- **#16 Token API exposure**: `tokens_in` / `tokens_out` on conversation + prompt serializers (null when unknown). `GET /api/finance/subscription/` returns plan + usage snapshot for the FE.
- Checkout defaults to **Founders**; Stripe paid webhooks assign Founders. Registration/OAuth redeem Backer whitelist and return `needs_checkout`.

Companion FE PR: `chat_web_app` branch `feature/plans-quotas-token-usage`.

## Test plan
- [ ] `manage.py migrate` seeds five plans; admin can add Backer emails
- [ ] Public `GET /api/finance/plans/` returns only Founders
- [ ] Register with Backer email → active Backer, `needs_checkout=false`, checkout rejected
- [ ] Founders checkout + paid webhook → active Founders subscription
- [ ] Chat turn blocked without subscription / when prompt window exceeded / when token period exceeded
- [ ] Standard plan denies image feature; Pro/Founders/Backer allow
- [ ] Conversation/prompt API returns `null` tokens when unreported, sums when present
- [ ] `finance.tests.test_plans_quotas` + existing finance/checkout tests passReviewed-on: #37

llm_be/finance/services/stripe_service.py

@@ -7,7 +7,8 @@ from typing import Any
 import stripe
 from django.conf import settings
 
-from finance.models import Invoice
+from finance.models import Invoice, SubscriptionPlan
+from finance.services.plans import get_plan, seed_subscription_plans
 
 
 class StripeNotConfiguredError(RuntimeError):
@@ -24,21 +25,47 @@ def configure_stripe() -> str:
     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
+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": settings.SUBSCRIPTION_PRICE_CURRENCY,
-                "unit_amount": settings.SUBSCRIPTION_PRICE_AMOUNT_CENTS,
-                "recurring": {"interval": settings.SUBSCRIPTION_PRICE_INTERVAL},
-                "product_data": {
-                    "name": settings.SUBSCRIPTION_PRODUCT_NAME,
-                },
+                "currency": currency,
+                "unit_amount": unit_amount,
+                "recurring": {"interval": interval},
+                "product_data": {"name": product_name},
             },
             "quantity": 1,
         }
@@ -50,19 +77,22 @@ 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 the subscription plan."""
+    """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(),
+        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,
@@ -70,7 +100,7 @@ def create_checkout_session(
         metadata=metadata,
         subscription_data={"metadata": metadata},
     )
-    return session
+    return session, plan
 
 
 def resolve_stripe_customer_id(*, user) -> str | None: