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/chat_backend/views_oauth.py

@@ -139,7 +139,9 @@ class OAuthCallbackView(APIView):
             return _redirect_error("server_error", "Unexpected OAuth error.")
 
         refresh = RefreshToken.for_user(user)
-        needs_checkout = "1" if created else "0"
+        from finance.services.plans import needs_checkout as user_needs_checkout
+
+        needs_checkout = "1" if (created and user_needs_checkout(user)) else "0"
         return HttpResponseRedirect(
             _frontend_callback_url(
                 access=str(refresh.access_token),