Add multi-plan subscriptions, quotas, and token usage APIs

Implements #16/#17/#36: Founders/Standard/Pro/Business/Backer catalog,
Backer email whitelist, prompt-window + token-period gates, and
tokens_in/out on conversation/prompt + subscription usage APIs.

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,
+        }