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/admin.py

@@ -1,6 +1,52 @@
 from django.contrib import admin
 
-from finance.models import Invoice, Payment
+from finance.models import BackerEmail, Invoice, Payment, SubscriptionPlan, UserSubscription
+
+
+@admin.register(SubscriptionPlan)
+class SubscriptionPlanAdmin(admin.ModelAdmin):
+    list_display = (
+        "slug",
+        "name",
+        "price_cents",
+        "is_public",
+        "is_selectable",
+        "allows_image_generation",
+        "allows_all_future_features",
+        "prompt_quota_per_window",
+        "prompt_window_hours",
+        "monthly_token_quota",
+        "sort_order",
+    )
+    list_filter = ("is_public", "is_selectable", "allows_image_generation")
+    search_fields = ("slug", "name", "stripe_price_id")
+    readonly_fields = ("created", "last_modified")
+    prepopulated_fields = {"slug": ("name",)}
+
+
+@admin.register(BackerEmail)
+class BackerEmailAdmin(admin.ModelAdmin):
+    list_display = ("email", "note", "redeemed_at", "redeemed_user", "created")
+    search_fields = ("email", "note", "redeemed_user__email")
+    raw_id_fields = ("redeemed_user",)
+    readonly_fields = ("created", "last_modified", "redeemed_at", "redeemed_user")
+
+
+@admin.register(UserSubscription)
+class UserSubscriptionAdmin(admin.ModelAdmin):
+    list_display = (
+        "user",
+        "plan",
+        "status",
+        "source",
+        "stripe_subscription_id",
+        "monthly_token_quota_override",
+        "created",
+    )
+    list_filter = ("status", "source", "plan")
+    search_fields = ("user__email", "user__username", "stripe_subscription_id")
+    raw_id_fields = ("user", "plan")
+    readonly_fields = ("created", "last_modified")
 
 
 class PaymentInline(admin.TabularInline):