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/hooks/useEntitlements.ts

@@ -0,0 +1,132 @@
+/**
+ * Entitlements Hook
+ *
+ * Provides utilities for checking feature availability based on the new billing system.
+ * This replaces the legacy usePlanFeatures hook for new billing-aware features.
+ */
+
+import { useQuery } from '@tanstack/react-query';
+import { getEntitlements, Entitlements } from '../api/billing';
+
+export interface UseEntitlementsResult {
+  /**
+   * The raw entitlements map
+   */
+  entitlements: Entitlements;
+
+  /**
+   * Whether entitlements are still loading
+   */
+  isLoading: boolean;
+
+  /**
+   * Check if a boolean feature is enabled
+   */
+  hasFeature: (featureCode: string) => boolean;
+
+  /**
+   * Get the limit value for an integer feature
+   * Returns null if the feature doesn't exist or is not an integer
+   */
+  getLimit: (featureCode: string) => number | null;
+
+  /**
+   * Refetch entitlements
+   */
+  refetch: () => void;
+}
+
+/**
+ * Hook to access entitlements from the billing system.
+ *
+ * Usage:
+ * ```tsx
+ * const { hasFeature, getLimit, isLoading } = useEntitlements();
+ *
+ * if (hasFeature('can_use_sms_reminders')) {
+ *   // Show SMS feature
+ * }
+ *
+ * const maxUsers = getLimit('max_users');
+ * if (maxUsers !== null && currentUsers >= maxUsers) {
+ *   // Show upgrade prompt
+ * }
+ * ```
+ */
+export const useEntitlements = (): UseEntitlementsResult => {
+  const { data, isLoading, refetch } = useQuery<Entitlements>({
+    queryKey: ['entitlements'],
+    queryFn: getEntitlements,
+    staleTime: 5 * 60 * 1000, // 5 minutes
+    retry: 1,
+  });
+
+  const entitlements = data ?? {};
+
+  /**
+   * Check if a boolean feature is enabled.
+   */
+  const hasFeature = (featureCode: string): boolean => {
+    const value = entitlements[featureCode];
+    return value === true;
+  };
+
+  /**
+   * Get the limit value for an integer feature.
+   * Returns null if the feature doesn't exist or is a boolean.
+   */
+  const getLimit = (featureCode: string): number | null => {
+    const value = entitlements[featureCode];
+    // Use strict type check to distinguish integers from booleans
+    // (typeof true === 'number' is false, but just to be safe)
+    if (typeof value === 'number' && !Number.isNaN(value)) {
+      return value;
+    }
+    return null;
+  };
+
+  return {
+    entitlements,
+    isLoading,
+    hasFeature,
+    getLimit,
+    refetch: () => refetch(),
+  };
+};
+
+/**
+ * Feature code constants for type safety
+ */
+export const FEATURE_CODES = {
+  // Boolean features (permissions)
+  CAN_ACCEPT_PAYMENTS: 'can_accept_payments',
+  CAN_USE_CUSTOM_DOMAIN: 'can_use_custom_domain',
+  CAN_WHITE_LABEL: 'can_white_label',
+  CAN_API_ACCESS: 'can_api_access',
+  CAN_USE_SMS_REMINDERS: 'can_use_sms_reminders',
+  CAN_USE_MASKED_PHONE_NUMBERS: 'can_use_masked_phone_numbers',
+  CAN_USE_MOBILE_APP: 'can_use_mobile_app',
+  CAN_USE_CONTRACTS: 'can_use_contracts',
+  CAN_USE_CALENDAR_SYNC: 'can_use_calendar_sync',
+  CAN_USE_WEBHOOKS: 'can_use_webhooks',
+  CAN_USE_PLUGINS: 'can_use_plugins',
+  CAN_USE_TASKS: 'can_use_tasks',
+  CAN_CREATE_PLUGINS: 'can_create_plugins',
+  CAN_EXPORT_DATA: 'can_export_data',
+  CAN_ADD_VIDEO_CONFERENCING: 'can_add_video_conferencing',
+  CAN_BOOK_REPEATED_EVENTS: 'can_book_repeated_events',
+  CAN_REQUIRE_2FA: 'can_require_2fa',
+  CAN_DOWNLOAD_LOGS: 'can_download_logs',
+  CAN_DELETE_DATA: 'can_delete_data',
+  CAN_USE_POS: 'can_use_pos',
+  CAN_MANAGE_OAUTH_CREDENTIALS: 'can_manage_oauth_credentials',
+  CAN_CONNECT_TO_API: 'can_connect_to_api',
+
+  // Integer features (limits)
+  MAX_USERS: 'max_users',
+  MAX_RESOURCES: 'max_resources',
+  MAX_EVENT_TYPES: 'max_event_types',
+  MAX_CALENDARS_CONNECTED: 'max_calendars_connected',
+} as const;
+
+export type FeatureCode = (typeof FEATURE_CODES)[keyof typeof FEATURE_CODES];