feat: add invite registration links with configurable usage limits

Admins can create one-time registration links (1–5× or unlimited uses)
with optional expiry (1d–14d or never). Recipients can register even
when public registration is disabled. Atomic usage counting prevents
race conditions, all endpoints are rate-limited.

client/src/App.tsx

@@ -107,7 +107,7 @@ export default function App() {
       <Routes>
         <Route path="/" element={<RootRedirect />} />
         <Route path="/login" element={<LoginPage />} />
-        <Route path="/register" element={<Navigate to="/login" replace />} />
+        <Route path="/register" element={<LoginPage />} />
         <Route
           path="/dashboard"
           element={