feat: Store next sample number on user collection

- Add nextSampleNumber field to AppUser record with default 100000
- Add compact constructor to initialize nextSampleNumber to 100000
- Add reserveNextSampleNumber method to atomically reserve and increment
- Update createSample to use reserveNextSampleNumber from user
- Update nextSampleNumber to read from user collection
- Update dashboardOverview to use new nextSampleNumber method

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

@@ -22,7 +22,13 @@ public record AppUser(
         String passwordHash,
         boolean active,
         UserRole role,
+        Long nextSampleNumber,
         LocalDateTime createdAt,
         LocalDateTime updatedAt
 ) {
+    public AppUser {
+        if (nextSampleNumber == null) {
+            nextSampleNumber = 100000L;
+        }
+    }
 }

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

@@ -68,8 +68,7 @@ public class SampleService {
                 .filter(sample -> !sample.completedAt().isBefore(today.atStartOfDay()))
                 .filter(sample -> sample.completedAt().isBefore(today.plusDays(1).atStartOfDay()))
                 .count();
-        String ownerAccountId = authorizationService.accountId(actor);
-        return new DashboardOverview(nextSampleNumber(ownerAccountId), openCount, completedToday, recent);
+        return new DashboardOverview(nextSampleNumber(actorId), openCount, completedToday, recent);
     }
 
     public LookupResult lookup(String actorId, long sampleNumber) {
@@ -106,10 +105,10 @@ public class SampleService {
                 .findFirst()
                 .orElseThrow(() -> new ResponseStatusException(HttpStatus.BAD_REQUEST, "Landwirt nicht gefunden"));
 
-        String ownerAccountId = authorizationService.accountId(actor);
+        long sampleNumber = reserveNextSampleNumber(actorId);
         Sample sample = new Sample(
                 null,
-                nextSampleNumber(ownerAccountId),
+                sampleNumber,
                 farmer.businessKey(),
                 farmer.name(),
                 farmer.email(),
@@ -519,16 +518,41 @@ public class SampleService {
         return nextSampleNumber(null);
     }
 
-    public long nextSampleNumber(String ownerAccountId) {
-        if (ownerAccountId != null) {
-            Optional<Sample> lastSample = sampleRepository.findTopByOwnerAccountIdOrderBySampleNumberDesc(ownerAccountId);
-            if (lastSample.isPresent()) {
-                return lastSample.get().sampleNumber() + 1;
-            }
+    public long nextSampleNumber(String actorId) {
+        if (actorId == null) {
+            return 100000L;
         }
-        return sampleRepository.findTopByOrderBySampleNumberDesc()
-                .map(sample -> sample.sampleNumber() + 1)
-                .orElse(10000L);
+        AppUser actor = requireActor(actorId);
+        return actor.nextSampleNumber() != null ? actor.nextSampleNumber() : 100000L;
+    }
+
+    public long reserveNextSampleNumber(String actorId) {
+        AppUser actor = requireActor(actorId);
+        long sampleNumber = actor.nextSampleNumber() != null ? actor.nextSampleNumber() : 100000L;
+        
+        // Update user with next sample number
+        appUserRepository.save(new AppUser(
+                actor.id(),
+                actor.accountId(),
+                actor.primaryUser(),
+                actor.displayName(),
+                actor.companyName(),
+                actor.address(),
+                actor.street(),
+                actor.houseNumber(),
+                actor.postalCode(),
+                actor.city(),
+                actor.email(),
+                actor.phoneNumber(),
+                actor.passwordHash(),
+                actor.active(),
+                actor.role(),
+                sampleNumber + 1,
+                actor.createdAt(),
+                LocalDateTime.now()
+        ));
+        
+        return sampleNumber;
     }
 
     public Sample loadSampleEntity(String actorId, String id) {