Introduce spice_stock_movements ledger for traceable stock changes

Every delivery and batch now writes a signed movement entry instead of
directly incrementing/decrementing spice.quantity. The balance is always
the sum of all movements. Existing data is backfilled in the migration.
Rohstoff edit page shows the full movement history (Lagerbewegungen tab).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Spice.php

@@ -14,4 +14,14 @@ class Spice extends Model
                     ->withPivot('amount_in_percent')
                     ->withTimestamps();
     }
+
+    public function stockMovements()
+    {
+        return $this->hasMany(SpiceStockMovement::class);
+    }
+
+    public function recalculateQuantity(): void
+    {
+        $this->update(['quantity' => $this->stockMovements()->sum('amount')]);
+    }
 }