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

backend/src/main/java/de/svencarstensen/muh/domain/SystemPricing.java

@@ -0,0 +1,15 @@
+package de.svencarstensen.muh.domain;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.mapping.Document;
+
+import java.time.LocalDateTime;
+
+@Document("systemPricing")
+public record SystemPricing(
+        @Id String id,
+        Double monthlyPrice,
+        LocalDateTime createdAt,
+        LocalDateTime updatedAt
+) {
+}

backend/src/main/java/de/svencarstensen/muh/repository/SystemPricingRepository.java

@@ -0,0 +1,7 @@
+package de.svencarstensen.muh.repository;
+
+import de.svencarstensen.muh.domain.SystemPricing;
+import org.springframework.data.mongodb.repository.MongoRepository;
+
+public interface SystemPricingRepository extends MongoRepository<SystemPricing, String> {
+}

backend/src/main/java/de/svencarstensen/muh/service/SystemPricingService.java

@@ -0,0 +1,38 @@
+package de.svencarstensen.muh.service;
+
+import de.svencarstensen.muh.domain.SystemPricing;
+import de.svencarstensen.muh.repository.SystemPricingRepository;
+import org.springframework.stereotype.Service;
+
+import java.time.LocalDateTime;
+import java.util.Optional;
+
+@Service
+public class SystemPricingService {
+
+    private static final String PRICING_ID = "default";
+
+    private final SystemPricingRepository systemPricingRepository;
+
+    public SystemPricingService(SystemPricingRepository systemPricingRepository) {
+        this.systemPricingRepository = systemPricingRepository;
+    }
+
+    public Optional<SystemPricing> getCurrentPricing() {
+        return systemPricingRepository.findById(PRICING_ID);
+    }
+
+    public SystemPricing savePricing(Double monthlyPrice) {
+        LocalDateTime now = LocalDateTime.now();
+        Optional<SystemPricing> existing = systemPricingRepository.findById(PRICING_ID);
+        
+        SystemPricing pricing = new SystemPricing(
+                PRICING_ID,
+                monthlyPrice,
+                existing.map(SystemPricing::createdAt).orElse(now),
+                now
+        );
+        
+        return systemPricingRepository.save(pricing);
+    }
+}

backend/src/main/java/de/svencarstensen/muh/web/SystemPricingController.java

@@ -0,0 +1,39 @@
+package de.svencarstensen.muh.web;
+
+import de.svencarstensen.muh.domain.SystemPricing;
+import de.svencarstensen.muh.service.SystemPricingService;
+import org.springframework.security.access.prepost.PreAuthorize;
+import org.springframework.web.bind.annotation.*;
+
+import java.util.Optional;
+
+@RestController
+@RequestMapping("/api/admin/pricing")
+@PreAuthorize("hasRole('ADMIN')")
+public class SystemPricingController {
+
+    private final SystemPricingService systemPricingService;
+
+    public SystemPricingController(SystemPricingService systemPricingService) {
+        this.systemPricingService = systemPricingService;
+    }
+
+    @GetMapping
+    public PricingResponse getPricing() {
+        Optional<SystemPricing> pricing = systemPricingService.getCurrentPricing();
+        return pricing.map(p -> new PricingResponse(p.monthlyPrice(), p.updatedAt().toString()))
+                .orElseGet(() -> new PricingResponse(null, null));
+    }
+
+    @PostMapping
+    public PricingResponse savePricing(@RequestBody PricingRequest request) {
+        SystemPricing saved = systemPricingService.savePricing(request.monthlyPrice());
+        return new PricingResponse(saved.monthlyPrice(), saved.updatedAt().toString());
+    }
+
+    public record PricingRequest(Double monthlyPrice) {
+    }
+
+    public record PricingResponse(Double monthlyPrice, String updatedAt) {
+    }
+}

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 />} />

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">

frontend/src/pages/PricingPage.tsx

@@ -0,0 +1,160 @@
+import { useEffect, useState } from "react";
+import { apiGet, apiPost } from "../lib/api";
+
+interface PricingResponse {
+  monthlyPrice: number | null;
+  updatedAt: string | null;
+}
+
+export default function PricingPage() {
+  const [monthlyPrice, setMonthlyPrice] = useState<string>("");
+  const [loading, setLoading] = useState(true);
+  const [saving, setSaving] = useState(false);
+  const [message, setMessage] = useState<string | null>(null);
+  const [lastUpdated, setLastUpdated] = useState<string | null>(null);
+
+  useEffect(() => {
+    async function loadPricing() {
+      try {
+        const response = await apiGet<PricingResponse>("/admin/pricing");
+        if (response.monthlyPrice !== null) {
+          setMonthlyPrice(response.monthlyPrice.toString());
+        }
+        setLastUpdated(response.updatedAt);
+      } catch (error) {
+        setMessage((error as Error).message);
+      } finally {
+        setLoading(false);
+      }
+    }
+
+    void loadPricing();
+  }, []);
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+
+    const priceValue = parseFloat(monthlyPrice.replace(",", "."));
+    if (isNaN(priceValue) || priceValue < 0) {
+      setMessage("Bitte geben Sie einen gültigen Preis ein.");
+      return;
+    }
+
+    setSaving(true);
+    setMessage(null);
+
+    try {
+      const response = await apiPost<PricingResponse>("/admin/pricing", {
+        monthlyPrice: priceValue,
+      });
+      setMonthlyPrice(response.monthlyPrice?.toString() ?? "");
+      setLastUpdated(response.updatedAt);
+      setMessage("Preis wurde erfolgreich gespeichert.");
+      setTimeout(() => setMessage(null), 3000);
+    } catch (error) {
+      setMessage((error as Error).message);
+    } finally {
+      setSaving(false);
+    }
+  }
+
+  function formatDate(value: string | null): string {
+    if (!value) return "-";
+    return new Intl.DateTimeFormat("de-DE", {
+      dateStyle: "medium",
+      timeStyle: "short",
+    }).format(new Date(value));
+  }
+
+  if (loading) {
+    return (
+      <div className="page-stack">
+        <section className="section-card">
+          <div className="empty-state">Preistabelle wird geladen...</div>
+        </section>
+      </div>
+    );
+  }
+
+  return (
+    <div className="page-stack">
+      {/* Header */}
+      <section className="hero-card admin-hero">
+        <div>
+          <p className="eyebrow">Preistabelle</p>
+          <h3>Monatlichen Systempreis festlegen</h3>
+          <p className="muted-text">
+            Legen Sie hier den monatlichen Preis für die Nutzung des Systems fest.
+            Dieser Preis wird für die Rechnungsstellung verwendet.
+          </p>
+        </div>
+      </section>
+
+      {/* Status-Meldung */}
+      {message && (
+        <div
+          className={
+            message.includes("erfolgreich")
+              ? "alert alert--success"
+              : "alert alert--error"
+          }
+        >
+          {message}
+        </div>
+      )}
+
+      {/* Preis-Formular */}
+      <section className="section-card">
+        <div className="section-card__header">
+          <div>
+            <p className="eyebrow">Systempreis</p>
+            <h3>Monatlicher Preis</h3>
+          </div>
+        </div>
+
+        <form onSubmit={handleSubmit} className="field-grid field-grid--2col">
+          <label className="field">
+            <span>Preis pro Monat (€) *</span>
+            <input
+              type="text"
+              value={monthlyPrice}
+              onChange={(e) => setMonthlyPrice(e.target.value)}
+              placeholder="z.B. 49,99"
+              required
+              disabled={saving}
+            />
+          </label>
+
+          <div className="field" style={{ display: "flex", alignItems: "flex-end" }}>
+            <button
+              type="submit"
+              className="accent-button"
+              disabled={saving}
+              style={{ width: "100%" }}
+            >
+              {saving ? "Wird gespeichert..." : "Preis speichern"}
+            </button>
+          </div>
+        </form>
+
+        {lastUpdated && (
+          <div style={{ marginTop: "1rem", fontSize: "0.875rem", color: "var(--muted-text)" }}>
+            <strong>Zuletzt aktualisiert:</strong> {formatDate(lastUpdated)}
+          </div>
+        )}
+      </section>
+
+      {/* Info-Box */}
+      <section className="section-card">
+        <div className="info-panel">
+          <strong>Hinweis</strong>
+          <p>
+            Der hier festgelegte monatliche Preis wird als Grundlage für die Abrechnung
+            mit den Nutzern verwendet. Änderungen werden sofort wirksam und bei der
+            nächsten Rechnungsstellung berücksichtigt.
+          </p>
+        </div>
+      </section>
+    </div>
+  );
+}