feat(contracts): Add contracts permission to subscription tiers

- Add contracts_enabled field to SubscriptionPlan model
- Add contracts toggle to plan create/edit modal in platform settings
- Hide contracts menu item for tenants without contracts permission
- Protect /contracts routes with canUse('contracts') check
- Add HasContractsPermission to contracts API ViewSets
- Add contracts to PlanPermissions interface and feature definitions

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

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

frontend/src/components/Sidebar.tsx

@@ -162,12 +162,14 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
                   label={t('nav.staff')}
                   isCollapsed={isCollapsed}
                 />
-                <SidebarItem
-                  to="/contracts"
-                  icon={FileSignature}
-                  label={t('nav.contracts', 'Contracts')}
-                  isCollapsed={isCollapsed}
-                />
+                {canUse('contracts') && (
+                  <SidebarItem
+                    to="/contracts"
+                    icon={FileSignature}
+                    label={t('nav.contracts', 'Contracts')}
+                    isCollapsed={isCollapsed}
+                  />
+                )}
                 <SidebarItem
                   to="/time-blocks"
                   icon={CalendarOff}