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/SpiceStockMovement.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class SpiceStockMovement extends Model
+{
+    protected $fillable = ['spice_id', 'amount', 'source_type', 'source_id', 'description'];
+
+    public function spice()
+    {
+        return $this->belongsTo(Spice::class);
+    }
+}