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/app/api/v1/documents/feature-flags/route.ts

@@ -0,0 +1,20 @@
+import { NextResponse } from 'next/server';
+
+import { withAuth, withPermission } from '@/lib/api/helpers';
+import { errorResponse } from '@/lib/errors';
+import { getSetting } from '@/lib/services/settings.service';
+
+export const GET = withAuth(
+  withPermission('documents', 'view', async (_req, ctx) => {
+    try {
+      const showExpired = await getSetting('documents_show_expired_tab', ctx.portId);
+      return NextResponse.json({
+        data: {
+          showExpiredTab: showExpired?.value !== false, // default true
+        },
+      });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  }),
+);