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/src/api/billing.ts

@@ -0,0 +1,184 @@
+/**
+ * Billing API
+ *
+ * API client functions for the billing/subscription system.
+ */
+
+import apiClient from './client';
+
+// ============================================================================
+// Types
+// ============================================================================
+
+/**
+ * Entitlements - a map of feature codes to their values.
+ * Boolean features indicate permission (true/false).
+ * Integer features indicate limits.
+ */
+export interface Entitlements {
+  [key: string]: boolean | number | null;
+}
+
+/**
+ * Plan information (nested in PlanVersion)
+ */
+export interface Plan {
+  code: string;
+  name: string;
+  description?: string;
+}
+
+/**
+ * Plan version with pricing and features
+ */
+export interface PlanVersion {
+  id: number;
+  name: string;
+  is_legacy: boolean;
+  is_public?: boolean;
+  plan: Plan;
+  price_monthly_cents: number;
+  price_yearly_cents: number;
+  features?: PlanFeature[];
+}
+
+/**
+ * Feature attached to a plan version
+ */
+export interface PlanFeature {
+  feature_code: string;
+  feature_name: string;
+  feature_type: 'boolean' | 'integer';
+  bool_value?: boolean;
+  int_value?: number;
+}
+
+/**
+ * Current subscription
+ */
+export interface Subscription {
+  id: number;
+  status: 'active' | 'canceled' | 'past_due' | 'trialing';
+  plan_version: PlanVersion;
+  current_period_start: string;
+  current_period_end: string;
+  canceled_at?: string;
+  stripe_subscription_id?: string;
+}
+
+/**
+ * Add-on product
+ */
+export interface AddOnProduct {
+  id: number;
+  code: string;
+  name: string;
+  description?: string;
+  price_monthly_cents: number;
+  is_active: boolean;
+}
+
+/**
+ * Invoice line item
+ */
+export interface InvoiceLine {
+  id: number;
+  line_type: 'plan' | 'addon' | 'adjustment' | 'credit';
+  description: string;
+  quantity: number;
+  unit_amount: number;
+  total_amount: number;
+}
+
+/**
+ * Invoice
+ */
+export interface Invoice {
+  id: number;
+  status: 'draft' | 'pending' | 'paid' | 'void' | 'uncollectible';
+  period_start: string;
+  period_end: string;
+  subtotal_amount: number;
+  total_amount: number;
+  plan_name_at_billing: string;
+  plan_code_at_billing?: string;
+  created_at: string;
+  paid_at?: string;
+  lines?: InvoiceLine[];
+}
+
+// ============================================================================
+// API Functions
+// ============================================================================
+
+/**
+ * Get effective entitlements for the current business.
+ * Returns a map of feature codes to their values.
+ */
+export const getEntitlements = async (): Promise<Entitlements> => {
+  try {
+    const response = await apiClient.get<Entitlements>('/me/entitlements/');
+    return response.data;
+  } catch (error) {
+    console.error('Failed to fetch entitlements:', error);
+    return {};
+  }
+};
+
+/**
+ * Get the current subscription for the business.
+ * Returns null if no subscription exists.
+ */
+export const getCurrentSubscription = async (): Promise<Subscription | null> => {
+  try {
+    const response = await apiClient.get<Subscription>('/me/subscription/');
+    return response.data;
+  } catch (error: any) {
+    if (error?.response?.status === 404) {
+      return null;
+    }
+    console.error('Failed to fetch subscription:', error);
+    throw error;
+  }
+};
+
+/**
+ * Get available plans (public, non-legacy plans).
+ */
+export const getPlans = async (): Promise<PlanVersion[]> => {
+  const response = await apiClient.get<PlanVersion[]>('/billing/plans/');
+  return response.data;
+};
+
+/**
+ * Get available add-on products.
+ */
+export const getAddOns = async (): Promise<AddOnProduct[]> => {
+  const response = await apiClient.get<AddOnProduct[]>('/billing/addons/');
+  return response.data;
+};
+
+/**
+ * Get invoices for the current business.
+ */
+export const getInvoices = async (): Promise<Invoice[]> => {
+  const response = await apiClient.get<Invoice[]>('/billing/invoices/');
+  return response.data;
+};
+
+/**
+ * Get a single invoice by ID.
+ * Returns null if not found.
+ */
+export const getInvoice = async (invoiceId: number): Promise<Invoice | null> => {
+  try {
+    const response = await apiClient.get<Invoice>(`/billing/invoices/${invoiceId}/`);
+    return response.data;
+  } catch (error: any) {
+    if (error?.response?.status === 404) {
+      return null;
+    }
+    console.error('Failed to fetch invoice:', error);
+    throw error;
+  }
+};