refactor(inventory): InvalidFilterCombination Error und CreateStockResponse trennen

- StockError.InvalidFilterCombination statt InvalidArticleId für
  ungültige Filter-Kombination in ListStocks
- CreateStockResponse als eigenständiges DTO für POST createStock
  (ohne batches/quantities), StockResponse bleibt für GET-Endpoints
- MinimumLevelResponse als shared Top-Level-Record extrahiert

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

@@ -22,7 +22,7 @@ public class ListStocks {
 
     public Result<StockError, List<Stock>> execute(String storageLocationId, String articleId) {
         if (storageLocationId != null && articleId != null) {
-            return Result.failure(new StockError.InvalidArticleId(
+            return Result.failure(new StockError.InvalidFilterCombination(
                     "Only one filter parameter allowed: storageLocationId or articleId"));
         }
 

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

@@ -80,6 +80,10 @@ public sealed interface StockError {
         @Override public String message() { return "Batch " + id + " is not blocked"; }
     }
 
+    record InvalidFilterCombination(String message) implements StockError {
+        @Override public String code() { return "INVALID_FILTER_COMBINATION"; }
+    }
+
     record Unauthorized(String message) implements StockError {
         @Override public String code() { return "UNAUTHORIZED"; }
     }

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

@@ -17,6 +17,7 @@ import de.effigenix.shared.security.ActorId;
 import de.effigenix.infrastructure.inventory.web.dto.AddStockBatchRequest;
 import de.effigenix.infrastructure.inventory.web.dto.BlockStockBatchRequest;
 import de.effigenix.infrastructure.inventory.web.dto.CreateStockRequest;
+import de.effigenix.infrastructure.inventory.web.dto.CreateStockResponse;
 import de.effigenix.infrastructure.inventory.web.dto.RemoveStockBatchRequest;
 import de.effigenix.infrastructure.inventory.web.dto.StockBatchResponse;
 import de.effigenix.infrastructure.inventory.web.dto.StockResponse;
@@ -93,7 +94,7 @@ public class StockController {
 
     @PostMapping
     @PreAuthorize("hasAuthority('STOCK_WRITE')")
-    public ResponseEntity<StockResponse> createStock(
+    public ResponseEntity<CreateStockResponse> createStock(
             @Valid @RequestBody CreateStockRequest request,
             Authentication authentication
     ) {
@@ -113,7 +114,7 @@ public class StockController {
 
         logger.info("Stock created: {}", result.unsafeGetValue().id().value());
         return ResponseEntity.status(HttpStatus.CREATED)
-                .body(StockResponse.from(result.unsafeGetValue()));
+                .body(CreateStockResponse.from(result.unsafeGetValue()));
     }
 
     @PostMapping("/{stockId}/batches")

backend/src/main/java/de/effigenix/infrastructure/inventory/web/dto/CreateStockResponse.java

@@ -0,0 +1,37 @@
+package de.effigenix.infrastructure.inventory.web.dto;
+
+import de.effigenix.domain.inventory.Stock;
+import io.swagger.v3.oas.annotations.media.Schema;
+
+@Schema(requiredProperties = {"id", "articleId", "storageLocationId"})
+public record CreateStockResponse(
+        String id,
+        String articleId,
+        String storageLocationId,
+        @Schema(nullable = true) MinimumLevelResponse minimumLevel,
+        @Schema(nullable = true) Integer minimumShelfLifeDays
+) {
+
+    public static CreateStockResponse from(Stock stock) {
+        MinimumLevelResponse minimumLevel = null;
+        if (stock.minimumLevel() != null) {
+            minimumLevel = new MinimumLevelResponse(
+                    stock.minimumLevel().quantity().amount(),
+                    stock.minimumLevel().quantity().uom().name()
+            );
+        }
+
+        Integer shelfLifeDays = null;
+        if (stock.minimumShelfLife() != null) {
+            shelfLifeDays = stock.minimumShelfLife().days();
+        }
+
+        return new CreateStockResponse(
+                stock.id().value(),
+                stock.articleId().value(),
+                stock.storageLocationId().value(),
+                minimumLevel,
+                shelfLifeDays
+        );
+    }
+}

backend/src/main/java/de/effigenix/infrastructure/inventory/web/dto/MinimumLevelResponse.java

@@ -0,0 +1,8 @@
+package de.effigenix.infrastructure.inventory.web.dto;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+import java.math.BigDecimal;
+
+@Schema(requiredProperties = {"amount", "unit"})
+public record MinimumLevelResponse(BigDecimal amount, String unit) {}

backend/src/main/java/de/effigenix/infrastructure/inventory/web/dto/StockResponse.java

@@ -63,7 +63,4 @@ public record StockResponse(
                 availableQuantity
         );
     }
-
-    @Schema(requiredProperties = {"amount", "unit"})
-    public record MinimumLevelResponse(BigDecimal amount, String unit) {}
 }

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

@@ -39,6 +39,7 @@ public final class InventoryErrorHttpStatusMapper {
             case StockError.InvalidBatchReference e -> 400;
             case StockError.InvalidQuantity e -> 400;
             case StockError.InvalidExpiryDate e -> 400;
+            case StockError.InvalidFilterCombination e -> 400;
             case StockError.Unauthorized e -> 403;
             case StockError.RepositoryFailure e -> 500;
         };

backend/src/test/java/de/effigenix/application/inventory/ListStocksTest.java

@@ -91,7 +91,7 @@ class ListStocksTest {
         var result = listStocks.execute("location-1", "article-1");
 
         assertThat(result.isFailure()).isTrue();
-        assertThat(result.unsafeGetError()).isInstanceOf(StockError.InvalidArticleId.class);
+        assertThat(result.unsafeGetError()).isInstanceOf(StockError.InvalidFilterCombination.class);
         verifyNoInteractions(stockRepository);
     }