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/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):