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) {
+    }
+}