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/layout/AppShell.tsx

@@ -8,6 +8,7 @@ const PAGE_TITLES: Record<string, string> = {
   "/samples/new": "Neuanlage einer Probe",
   "/portal": "MUH-Portal",
   "/report-template": "Bericht",
+  "/admin/preistabelle": "Preistabelle",
   "/admin/rechnung/verwalten": "Rechnungsverwaltung",
   "/admin/rechnung/template": "Rechnungsvorlage",
 };
@@ -64,6 +65,13 @@ export default function AppShell() {
                 </div>
               </div>
 
+              <NavLink
+                to="/admin/preistabelle"
+                className={({ isActive }) => `nav-link ${isActive ? "is-active" : ""}`}
+              >
+                Preistabelle
+              </NavLink>
+
               <div className="nav-group">
                 <div className="nav-group__label">Rechnung</div>
                 <div className="nav-subnav">