fix: answer 503 for plugin lists and marketplace installs while the shared store is down #1066

directory-keys-shared.test.ts: the store-down case now expects the store error instead of an 'invalid' signature, since a store outage no longer counts as a bad signature.

app/api/admin/plugins/route.ts

@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from 'next/server';
 import { requireAdminAuth, getClientIP } from '@/lib/admin/session';
 import { auditLog } from '@/lib/admin/audit';
 import { logger } from '@/lib/logger';
+import { storeUnavailableResponse } from '@/lib/admin/store-unavailable-response';
 import {
   getPlugin,
   getPluginRegistry,
@@ -46,6 +47,8 @@ export async function GET(request: NextRequest) {
       headers: { 'Cache-Control': 'no-store' },
     });
   } catch (error) {
+    const unavailable = storeUnavailableResponse(error);
+    if (unavailable) return unavailable;
     logger.error('Plugin list error', { error: error instanceof Error ? error.message : 'Unknown error' });
     return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
   }