fix(inventory): confirmReservation – Layering, Error-Semantik und FK-Bug

- MovementType-Ableitung via ReferenceType.toMovementType() in Domain Layer
- Doppelten Batch-Lookup in Stock.confirmReservation() eliminiert
- StockError.MovementCreationFailed statt RepositoryFailure für Domain-Fehler
- 204 No Content statt 200 OK (konsistent mit releaseReservation)
- Batches mit Menge 0 nicht mehr entfernen (FK stock_movements → stock_batches)

backend/src/main/java/de/effigenix/application/inventory/ConfirmReservation.java

@@ -44,10 +44,7 @@ public class ConfirmReservation {
         }
 
         // 3. MovementType aus ReferenceType ableiten
-        String movementType = switch (confirmed.referenceType()) {
-            case PRODUCTION_ORDER -> "PRODUCTION_CONSUMPTION";
-            case SALE_ORDER -> "SALE";
-        };
+        String movementType = confirmed.referenceType().toMovementType().name();
 
         // 4. StockMovements erzeugen
         List<StockMovement> movements = new ArrayList<>();
@@ -68,7 +65,7 @@ public class ConfirmReservation {
             );
             switch (StockMovement.record(draft)) {
                 case Result.Failure(var err) ->
-                        { return Result.failure(new StockError.RepositoryFailure(err.message())); }
+                        { return Result.failure(new StockError.MovementCreationFailed(err.message())); }
                 case Result.Success(var val) -> movements.add(val);
             }
         }

backend/src/main/java/de/effigenix/domain/inventory/ReferenceType.java

@@ -2,5 +2,12 @@ package de.effigenix.domain.inventory;
 
 public enum ReferenceType {
     PRODUCTION_ORDER,
-    SALE_ORDER
+    SALE_ORDER;
+
+    public MovementType toMovementType() {
+        return switch (this) {
+            case PRODUCTION_ORDER -> MovementType.PRODUCTION_CONSUMPTION;
+            case SALE_ORDER -> MovementType.SALE;
+        };
+    }
 }

backend/src/main/java/de/effigenix/domain/inventory/Stock.java

@@ -35,7 +35,7 @@ import java.util.stream.Collectors;
  * - reserve: FEFO allocation across AVAILABLE/EXPIRING_SOON batches sorted by expiryDate ASC
  * - releaseReservation: removes reservation by ID, implicitly freeing allocated quantities
  * - confirmReservation: deducts allocated quantities from batches, removes reservation, returns ConfirmedReservation
- *   - batches with quantity 0 after deduction are removed
+ *   - batches with quantity 0 after deduction are kept (FK from stock_movements requires batch persistence)
  * - reservations track allocated quantities per batch; no over-reservation possible
  */
 public class Stock {
@@ -366,7 +366,7 @@ public class Stock {
             return Result.failure(new StockError.ReservationNotFound(reservationId.value()));
         }
 
-        // 2. Für jede Allocation: Batch finden und BatchReference merken
+        // 2. Für jede Allocation: Batch finden, Menge abziehen, ConfirmedAllocation erzeugen
         List<ConfirmedAllocation> confirmedAllocations = new ArrayList<>();
         for (StockBatchAllocation alloc : reservation.allocations()) {
             StockBatch batch = batches.stream()
@@ -376,35 +376,21 @@ public class Stock {
             if (batch == null) {
                 return Result.failure(new StockError.BatchNotFound(alloc.stockBatchId().value()));
             }
+
             confirmedAllocations.add(new ConfirmedAllocation(
                     batch.id(), batch.batchReference(), alloc.allocatedQuantity()));
-        }
-
-        // 3. Für jede Allocation: Menge abziehen, bei 0 Batch entfernen
-        for (ConfirmedAllocation confirmed : confirmedAllocations) {
-            StockBatch batch = batches.stream()
-                    .filter(b -> b.id().equals(confirmed.stockBatchId()))
-                    .findFirst()
-                    .orElse(null);
-            if (batch == null) {
-                return Result.failure(new StockError.BatchNotFound(confirmed.stockBatchId().value()));
-            }
 
             Quantity remaining;
-            switch (batch.removeQuantity(confirmed.allocatedQuantity())) {
+            switch (batch.removeQuantity(alloc.allocatedQuantity())) {
                 case Result.Failure(var err) -> { return Result.failure(err); }
                 case Result.Success(var val) -> remaining = val;
             }
 
-            if (remaining.amount().signum() == 0) {
-                this.batches.remove(batch);
-            } else {
-                int index = this.batches.indexOf(batch);
-                this.batches.set(index, batch.withQuantity(remaining));
-            }
+            int index = this.batches.indexOf(batch);
+            this.batches.set(index, batch.withQuantity(remaining));
         }
 
-        // 4. Reservation entfernen
+        // 3. Reservation entfernen
         this.reservations.remove(reservation);
 
         return Result.success(new ConfirmedReservation(

backend/src/main/java/de/effigenix/domain/inventory/StockError.java

@@ -113,6 +113,10 @@ public sealed interface StockError {
         @Override public String code() { return "UNAUTHORIZED"; }
     }
 
+    record MovementCreationFailed(String message) implements StockError {
+        @Override public String code() { return "MOVEMENT_CREATION_FAILED"; }
+    }
+
     record RepositoryFailure(String message) implements StockError {
         @Override public String code() { return "REPOSITORY_ERROR"; }
     }

backend/src/main/java/de/effigenix/infrastructure/inventory/web/controller/StockController.java

@@ -332,7 +332,7 @@ public class StockController {
         }
 
         logger.info("Reservation {} of stock {} confirmed", reservationId, stockId);
-        return ResponseEntity.ok().build();
+        return ResponseEntity.noContent().build();
     }
 
     public static class StockDomainErrorException extends RuntimeException {

backend/src/main/java/de/effigenix/infrastructure/inventory/web/exception/InventoryErrorHttpStatusMapper.java

@@ -47,6 +47,7 @@ public final class InventoryErrorHttpStatusMapper {
             case StockError.InvalidReferenceId e -> 400;
             case StockError.InvalidReservationPriority e -> 400;
             case StockError.Unauthorized e -> 403;
+            case StockError.MovementCreationFailed e -> 500;
             case StockError.RepositoryFailure e -> 500;
         };
     }