feat(inventory): Bestand reservieren mit FEFO-Allokation (#12)

Implementiert Story 4.1: Reservierung von Beständen mit automatischer
FEFO-Allokation (First-Expired-First-Out) über verfügbare Chargen.

Domain: Reservation-Entity, StockBatchAllocation, ReservationDraft,
FEFO-Logik in Stock.reserve(), availableQuantity() berücksichtigt
bestehende Allokationen. Neue Error-Varianten für InsufficientStock,
InvalidReferenceType, InvalidReservationPriority, ReservationNotFound.

API: POST /api/inventory/stocks/{stockId}/reservations → 201 Created.
Liquibase: reservations + stock_batch_allocations Tabellen mit FK- und
CHECK-Constraints.

Tests: 43 neue Tests (22 Domain, 10 UseCase, 11 Integration) für
FEFO-Logik, Validierung, Mengenprüfung, Auth und Edge Cases.

backend/docs/mvp/ddd/07-inventory-bc.md

@@ -91,6 +91,7 @@ classDiagram
     }
 
     class StockBatchAllocation {
+        +AllocationId id
         +StockBatchId stockBatchId
         +Quantity allocatedQuantity
     }
@@ -163,6 +164,8 @@ classDiagram
     StockBatch ..> StockBatchStatus
     StockBatch ..> Quantity
 
+    StockBatchAllocation ..> AllocationId
+
     StockMovement ..> StockMovementId
     StockMovement ..> MovementType
     StockMovement ..> MovementDirection
@@ -208,6 +211,7 @@ Stock (Aggregate Root)
     ├── Priority (ReservationPriority: URGENT | NORMAL | LOW)
     ├── ReservedAt (Instant)
     └── StockBatchAllocations[] (Entity)
+        ├── AllocationId (VO)
         ├── StockBatchId (VO) - Zugeordnete Charge
         └── AllocatedQuantity (VO) - Reservierte Menge aus dieser Charge
 ```
@@ -846,6 +850,15 @@ public sealed interface StockError {
     record NegativeStockNotAllowed() implements StockError {
         public String message() { return "Stock cannot go negative"; }
     }
+    record InvalidReferenceType(String value) implements StockError {
+        public String message() { return "Invalid reference type: " + value; }
+    }
+    record InvalidReferenceId(String reason) implements StockError {
+        public String message() { return "Invalid reference ID: " + reason; }
+    }
+    record InvalidReservationPriority(String value) implements StockError {
+        public String message() { return "Invalid reservation priority: " + value; }
+    }
 }
 
 public sealed interface StockMovementError {