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/hooks/useBusiness.ts

@@ -30,8 +30,8 @@ export const useCurrentBusiness = () => {
         id: String(data.id),
         name: data.name,
         subdomain: data.subdomain,
-        primaryColor: data.primary_color,
-        secondaryColor: data.secondary_color,
+        primaryColor: data.primary_color || '#3B82F6',  // Blue-500 default
+        secondaryColor: data.secondary_color || '#1E40AF',  // Blue-800 default
         logoUrl: data.logo_url,
         whitelabelEnabled: data.whitelabel_enabled,
         plan: data.tier, // Map tier to plan