Add comprehensive backend test suite (#339)

* add test suite, mostly covers integration testing, tests are only backend side

* workflow runs the correct script

* workflow runs the correct script

* workflow runs the correct script

* unit tests incoming

* Fix multer silent rejections and error handler info leak

- Revert cb(null, false) to cb(new Error(...)) in auth.ts, collab.ts,
  and files.ts so invalid uploads return an error instead of silently
  dropping the file
- Error handler in app.ts now always returns 500 / "Internal server
  error" instead of forwarding err.message to the client

* Use statusCode consistently for multer errors and error handler

- Error handler in app.ts reads err.statusCode to forward the correct
  HTTP status while keeping the response body generic

client/src/pages/AdminPage.tsx

@@ -18,7 +18,7 @@ import PackingTemplateManager from '../components/Admin/PackingTemplateManager'
 import AuditLogPanel from '../components/Admin/AuditLogPanel'
 import AdminMcpTokensPanel from '../components/Admin/AdminMcpTokensPanel'
 import PermissionsPanel from '../components/Admin/PermissionsPanel'
-import { Users, Map, Briefcase, Shield, Trash2, Edit2, Camera, FileText, Eye, EyeOff, Save, CheckCircle, XCircle, Loader2, UserPlus, ArrowUpCircle, ExternalLink, Download, GitBranch, Sun, Link2, Copy, Plus, RefreshCw, AlertTriangle } from 'lucide-react'
+import { Users, Map, Briefcase, Shield, Trash2, Edit2, FileText, Eye, EyeOff, Save, CheckCircle, XCircle, Loader2, UserPlus, ArrowUpCircle, ExternalLink, Download, Sun, Link2, Copy, Plus, RefreshCw, AlertTriangle } from 'lucide-react'
 import CustomSelect from '../components/shared/CustomSelect'
 
 interface AdminUser {