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>

database/migrations/2026_09_12_250000_create_spice_stock_movements.php

@@ -0,0 +1,67 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('spice_stock_movements', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('spice_id')->constrained('spices')->cascadeOnDelete();
+            $table->decimal('amount', 10, 2); // positiv = Zugang, negativ = Abgang
+            $table->enum('source_type', ['delivery', 'batch']);
+            $table->unsignedBigInteger('source_id');
+            $table->string('description');
+            $table->timestamps();
+
+            $table->index(['spice_id', 'created_at']);
+        });
+
+        // Bestehende Lieferungen als Bewegungen einlesen
+        DB::table('spices_suppliers')->orderBy('id')->each(function ($row) {
+            DB::table('spice_stock_movements')->insert([
+                'spice_id'    => $row->spice_id,
+                'amount'      => $row->amount,
+                'source_type' => 'delivery',
+                'source_id'   => $row->id,
+                'description' => 'Lieferung',
+                'created_at'  => $row->created_at,
+                'updated_at'  => $row->updated_at,
+            ]);
+        });
+
+        // Bestehende Chargen als Bewegungen einlesen
+        DB::table('batches')->orderBy('id')->each(function ($batch) {
+            $spices = DB::table('recipes_spices')->where('recipe_id', $batch->recipe_id)->get();
+            foreach ($spices as $pivot) {
+                $grams = $batch->quantity_g * $pivot->amount_in_percent / 100;
+                DB::table('spice_stock_movements')->insert([
+                    'spice_id'    => $pivot->spice_id,
+                    'amount'      => -$grams,
+                    'source_type' => 'batch',
+                    'source_id'   => $batch->id,
+                    'description' => 'Charge ' . $batch->lot_number,
+                    'created_at'  => $batch->created_at,
+                    'updated_at'  => $batch->updated_at,
+                ]);
+            }
+        });
+
+        // Bestand aller Rohstoffe aus den Bewegungen neu berechnen
+        DB::table('spices')->orderBy('id')->each(function ($spice) {
+            $balance = DB::table('spice_stock_movements')
+                ->where('spice_id', $spice->id)
+                ->sum('amount');
+            DB::table('spices')->where('id', $spice->id)->update(['quantity' => $balance]);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('spice_stock_movements');
+    }
+};