Add TenantCustomTier system and fix BusinessEditModal feature loading

Backend:
- Add TenantCustomTier model for per-tenant feature overrides
- Update EntitlementService to check custom tier before plan features
- Add custom_tier action on TenantViewSet (GET/PUT/DELETE)
- Add Celery task for grace period management (30-day expiry)

Frontend:
- Add DynamicFeaturesEditor component for dynamic feature management
- Fix BusinessEditModal to load features from plan defaults when no custom tier
- Update limits (max_users, max_resources, etc.) to use featureValues
- Remove outdated canonical feature check from FeaturePicker (removes warning icons)
- Add useBillingPlans hook for accessing billing system data
- Add custom tier API functions to platform.ts

Features now follow consistent rules:
- Load from plan defaults when no custom tier exists
- Load from custom tier when one exists
- Reset to plan defaults when plan changes
- Save to custom tier on edit

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

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

frontend/src/components/Sidebar.tsx

@@ -18,6 +18,7 @@ import {
   FileSignature,
   CalendarOff,
   LayoutTemplate,
+  MapPin,
 } from 'lucide-react';
 import { Business, User } from '../types';
 import { useLogout } from '../hooks/useAuth';
@@ -204,6 +205,13 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
                   label={t('nav.timeBlocks', 'Time Blocks')}
                   isCollapsed={isCollapsed}
                 />
+                <SidebarItem
+                  to="/locations"
+                  icon={MapPin}
+                  label={t('nav.locations', 'Locations')}
+                  isCollapsed={isCollapsed}
+                  locked={!canUse('multi_location')}
+                />
               </>
             )}
           </SidebarSection>