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/documents/documents-hub.tsx

@@ -119,6 +119,13 @@ export function DocumentsHub({ portSlug, initialTab = 'all' }: DocumentsHubProps
     staleTime: 30_000,
   });
 
+  const { data: flagsResp } = useQuery<{ data: { showExpiredTab: boolean } }>({
+    queryKey: ['documents', 'feature-flags'],
+    queryFn: () => apiFetch('/api/v1/documents/feature-flags'),
+    staleTime: 5 * 60 * 1000,
+  });
+  const showExpiredTab = flagsResp?.data.showExpiredTab ?? true;
+
   useRealtimeInvalidation({
     'document:created': [['documents']],
     'document:updated': [['documents']],
@@ -275,7 +282,7 @@ export function DocumentsHub({ portSlug, initialTab = 'all' }: DocumentsHubProps
           }}
         >
           <TabsList>
-            {documentsHubTabs.map((t) => (
+            {documentsHubTabs.filter((t) => t !== 'expired' || showExpiredTab).map((t) => (
               <TabsTrigger key={t} value={t}>
                 {TAB_LABELS[t]}
                 {t !== 'all' && counts[t] > 0 ? (