feat: Add Preistabelle for admin to manage monthly system price

- Add SystemPricing domain model to store monthly price in MongoDB
- Add SystemPricingRepository for database access
- Add SystemPricingService with get/save functionality
- Add SystemPricingController with GET/POST endpoints (admin only)
- Add PricingPage component for frontend
- Add navigation menu item for Preistabelle (above Rechnung)
- Add route /admin/preistabelle for the new page

frontend/src/App.tsx

@@ -17,6 +17,7 @@ import UserManagementPage from "./pages/UserManagementPage";
 import ReportTemplatePage from "./pages/ReportTemplatePage";
 import InvoiceTemplatePage from "./pages/InvoiceTemplatePage";
 import InvoiceManagementPage from "./pages/InvoiceManagementPage";
+import PricingPage from "./pages/PricingPage";
 
 function ProtectedRoutes() {
   const { user, ready } = useSession();
@@ -47,6 +48,7 @@ function ProtectedRoutes() {
         <Route path="/admin/medikamente" element={<AdministrationPage />} />
         <Route path="/admin/erreger" element={<AdministrationPage />} />
         <Route path="/admin/antibiogramm" element={<AdministrationPage />} />
+        <Route path="/admin/preistabelle" element={<PricingPage />} />
         <Route path="/admin/rechnung/verwalten" element={<InvoiceManagementPage />} />
         <Route path="/admin/rechnung/template" element={<InvoiceTemplatePage />} />
         <Route path="/search" element={<Navigate to="/search/probe" replace />} />