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/webhooks.py

@@ -11,6 +11,7 @@ from django.db import transaction
 from django.utils import timezone
 
 from finance.models import Invoice, Payment
+from finance.services.plans import assign_founders_from_stripe
 
 logger = logging.getLogger(__name__)
 User = get_user_model()
@@ -211,6 +212,11 @@ def handle_checkout_session_completed(session: dict[str, Any]) -> Invoice | None
             ),
             paid_at=timezone.now(),
         )
+    if session.get("payment_status") == "paid" or session.get("subscription"):
+        assign_founders_from_stripe(
+            user,
+            stripe_subscription_id=session.get("subscription") or "",
+        )
     return invoice
 
 
@@ -270,6 +276,10 @@ def handle_invoice_paid(stripe_invoice: dict[str, Any]) -> Invoice | None:
             stripe_charge_id=charge if isinstance(charge, str) else None,
             paid_at=paid_at,
         )
+    assign_founders_from_stripe(
+        user,
+        stripe_subscription_id=stripe_invoice.get("subscription") or "",
+    )
     return invoice