Add scheduler improvements, API endpoints, and month calendar view

Backend:
- Add /api/customers/ endpoint (CustomerViewSet, CustomerSerializer)
- Add /api/services/ endpoint with Service model and migrations
- Add Resource.type field (STAFF, ROOM, EQUIPMENT) with migration
- Fix EventSerializer to return resource_id, customer_id, service_id
- Add date range filtering to EventViewSet (start_date, end_date params)
- Add create_demo_appointments management command
- Set default brand colors in business API

Frontend:
- Add calendar grid view for month mode in OwnerScheduler
- Fix sidebar navigation active link contrast (bg-white/10)
- Add default primaryColor/secondaryColor fallbacks in useBusiness
- Disable WebSocket (backend not implemented) to stop reconnect loop
- Fix Resource.type.toLowerCase() error by adding type to backend

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

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

frontend/src/components/Sidebar.tsx

@@ -36,7 +36,7 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
 
     const baseClasses = `flex items-center gap-3 py-3 text-sm font-medium rounded-lg transition-colors`;
     const collapsedClasses = isCollapsed ? 'px-3 justify-center' : 'px-4';
-    const activeClasses = 'bg-opacity-10 text-white bg-white';
+    const activeClasses = 'bg-white/10 text-white';
     const inactiveClasses = 'text-white/70 hover:text-white hover:bg-white/5';
     const disabledClasses = 'text-white/30 cursor-not-allowed';