feat(documents): admin-configurable Expired tab visibility

New documents_show_expired_tab system setting (default true). Public
read via GET /api/v1/documents/feature-flags (gated on documents.view
so reps can read it without holding manage_settings). When off, the
Expired tab is hidden from the documents hub — useful when expired
EOIs are noise that distracts reps from active deals.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/settings/settings-manager.tsx

@@ -210,6 +210,14 @@ const KNOWN_SETTINGS: Array<{
     type: 'boolean',
     defaultValue: true,
   },
+  {
+    key: 'documents_show_expired_tab',
+    label: 'Documents — show Expired tab',
+    description:
+      'When off, the Expired tab on the documents hub is hidden. Use this when expired EOIs are noise that distracts reps from active deals.',
+    type: 'boolean',
+    defaultValue: true,
+  },
 ];
 
 export function SettingsManager() {