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/serializers.py

@@ -1,6 +1,6 @@
 from rest_framework import serializers
 
-from finance.models import Invoice, Payment
+from finance.models import Invoice, Payment, SubscriptionPlan
 
 
 class InvoiceSerializer(serializers.ModelSerializer):
@@ -49,7 +49,38 @@ class PaymentSerializer(serializers.ModelSerializer):
 class CheckoutSessionSerializer(serializers.Serializer):
     success_url = serializers.URLField(required=False, allow_blank=False)
     cancel_url = serializers.URLField(required=False, allow_blank=False)
+    plan_slug = serializers.SlugField(required=False, allow_blank=False)
 
 
 class PortalSessionSerializer(serializers.Serializer):
     return_url = serializers.URLField(required=False, allow_blank=False)
+
+
+class SubscriptionPlanSerializer(serializers.ModelSerializer):
+    features = serializers.SerializerMethodField()
+
+    class Meta:
+        model = SubscriptionPlan
+        fields = [
+            "slug",
+            "name",
+            "description",
+            "price_cents",
+            "currency",
+            "interval",
+            "is_public",
+            "is_selectable",
+            "features",
+            "prompt_quota_per_window",
+            "prompt_window_hours",
+            "monthly_token_quota",
+            "sort_order",
+        ]
+        read_only_fields = fields
+
+    def get_features(self, obj):
+        return {
+            "text_generation": obj.allows_feature("text_generation"),
+            "image_generation": obj.allows_feature("image_generation"),
+            "all_future_features": obj.allows_all_future_features,
+        }