feat: Add subscription/billing/entitlement system

Implements a complete billing system with:

Backend (Django):
- New billing app with models: Feature, Plan, PlanVersion, PlanFeature,
  Subscription, AddOnProduct, AddOnFeature, SubscriptionAddOn,
  EntitlementOverride, Invoice, InvoiceLine
- EntitlementService with resolution order: overrides > add-ons > plan
- Invoice generation service with immutable snapshots
- DRF API endpoints for entitlements, subscription, plans, invoices
- Data migrations to seed initial plans and convert existing tenants
- Bridge to legacy Tenant.has_feature() with fallback support
- 75 tests covering models, services, and API endpoints

Frontend (React):
- Billing API client (getEntitlements, getPlans, getInvoices, etc.)
- useEntitlements hook with hasFeature() and getLimit() helpers
- FeatureGate and LimitGate components for conditional rendering
- 29 tests for API, hook, and components

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/package.json

@@ -35,6 +35,7 @@
     "@eslint/js": "^9.39.1",
     "@playwright/test": "^1.48.0",
     "@tailwindcss/postcss": "^4.1.17",
+    "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.0",
     "@testing-library/user-event": "^14.6.1",