feat(frontend): slice 3 — Backups cluster migration + nav/IA

Web UI rework.
- Migrate BackupAlertsTable, BackupJobsTable, BackupRunsTable,
  BackupsPage, BackupDashboardWidget off @mui (shadcn Table + Badge
  severity variants: success=chart-2, warning=chart-3, destructive)
- App.tsx IA: Backups now top-level nav (DatabaseBackup icon);
  Media surface primary at /media; /applications -> /media redirect
  in both route trees (mirrors /monitoring -> /observability)

Gate: build + lint + test green.

frontend/src/App.tsx

@@ -43,6 +43,7 @@ import {
 import {
 	LayoutDashboard,
 	Activity,
+	DatabaseBackup,
 	Monitor,
 	Users,
 	Zap,
@@ -82,8 +83,9 @@ function useDarkMode() {
 const navItems = [
 	{ path: "/", label: "Dashboard", icon: LayoutDashboard },
 	{ path: "/observability", label: "Observability", icon: Activity },
-	{ path: "/applications", label: "Media", icon: Monitor },
+	{ path: "/media", label: "Media", icon: Monitor },
 	{ path: "/files", label: "Files", icon: FolderOpen },
+	{ path: "/backups", label: "Backups", icon: DatabaseBackup },
 	{ path: "/users", label: "Users", icon: Users },
 	{ path: "/actions", label: "Actions", icon: Zap },
 	{ path: "/settings", label: "Settings", icon: SettingsIcon },
@@ -432,9 +434,15 @@ function AppInner() {
 						<Routes>
 							<Route element={<AuthenticatedApp />}>
 								<Route path="/" element={<Dashboard />} />
-								<Route path="/monitoring" element={<Navigate to="/observability" replace />} />
-								<Route path="/applications" element={<Applications />} />
+								<Route
+									path="/monitoring"
+									element={<Navigate to="/observability" replace />}
+								/>
 								<Route path="/media" element={<Applications />} />
+								<Route
+									path="/applications"
+									element={<Navigate to="/media" replace />}
+								/>
 								<Route path="/users" element={<UsersPage />} />
 								<Route path="/actions" element={<Actions />} />
 								<Route path="/files" element={<FileBrowser />} />
@@ -457,9 +465,15 @@ function AppInner() {
 							}
 						>
 							<Route path="/" element={<Dashboard />} />
-							<Route path="/monitoring" element={<Navigate to="/observability" replace />} />
-							<Route path="/applications" element={<Applications />} />
+							<Route
+								path="/monitoring"
+								element={<Navigate to="/observability" replace />}
+							/>
 							<Route path="/media" element={<Applications />} />
+							<Route
+								path="/applications"
+								element={<Navigate to="/media" replace />}
+							/>
 							<Route path="/users" element={<UsersPage />} />
 							<Route path="/actions" element={<Actions />} />
 							<Route path="/files" element={<FileBrowser />} />