feat: version guard — prompt stale clients to refresh after deploys

- Inject NEXT_PUBLIC_BUILD_ID at build time via next.config.ts
- /api/version static route returns current build ID
- VersionGuard client component checks on tab focus + every 5 min
- Shows persistent toast with Refresh button (no auto-reload)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

next.config.ts

@@ -2,6 +2,9 @@ import type { NextConfig } from 'next'
 
 const nextConfig: NextConfig = {
   output: 'standalone',
+  env: {
+    NEXT_PUBLIC_BUILD_ID: Date.now().toString(),
+  },
   serverExternalPackages: ['@prisma/client', 'minio'],
   typescript: {
     // We run tsc --noEmit separately before each push