feat(admin): System health page surfacing stuck/failed emails

New /admin/system-health page (sidebar entry, settings.view) that lists
emails the queue gave up on (status='failed' or pending+retry>=3) with
retry (re-queue) and dismiss (delete) actions. Backend adds /failures,
/failures/email/:id/retry and DELETE on adminSystemHealth. First source
is email failures (the original trigger — quote_sent template errors
left invoices unsent for 14h with no signal); more sources can be added.

frontend/src/components/admin/AdminSidebar.tsx

@@ -6,6 +6,7 @@ import {
   Archive,
   BarChart3,
   Settings,
+  Activity,
   X,
   Users,
   Briefcase,
@@ -62,6 +63,7 @@ const navigation: NavItem[] = [
   { nameKey: 'navigation.archives',  href: '/admin/archives',  icon: Archive,         permission: 'archives.view' },
   { nameKey: 'admin.analytics',      href: '/admin/analytics', icon: BarChart3,       permission: 'analytics.view', featureFlag: 'analytics' },
   { nameKey: 'navigation.settings',  href: '/admin/settings',  icon: Settings,        permission: 'settings.view' },
+  { nameKey: 'navigation.systemHealth', href: '/admin/system-health', icon: Activity,  permission: 'settings.view' },
   { nameKey: 'navigation.users',     href: '/admin/users',     icon: Users,           permission: 'users.view',     featureFlag: 'userManagement' },
   // Clients section (#354 follow-up) — admin-side surface for the
   // CRM-area sub-features. Today this entry leads to /admin/clients