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/Filament/User/Resources/SpiceResource/RelationManagers/StockMovementsRelationManager.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Filament\User\Resources\SpiceResource\RelationManagers;
+
+use Filament\Resources\RelationManagers\RelationManager;
+use Filament\Tables;
+use Filament\Tables\Table;
+
+class StockMovementsRelationManager extends RelationManager
+{
+    protected static string $relationship = 'stockMovements';
+    protected static ?string $title = 'Lagerbewegungen';
+
+    public function table(Table $table): Table
+    {
+        return $table
+            ->columns([
+                Tables\Columns\TextColumn::make('created_at')
+                    ->label('Datum')
+                    ->dateTime('d.m.Y H:i')
+                    ->sortable(),
+
+                Tables\Columns\TextColumn::make('description')
+                    ->label('Beschreibung'),
+
+                Tables\Columns\TextColumn::make('amount')
+                    ->label('Menge')
+                    ->formatStateUsing(fn ($state) => ($state > 0 ? '+' : '') . number_format($state, 2, ',', '.') . 'g')
+                    ->color(fn ($state) => $state > 0 ? 'success' : 'danger'),
+            ])
+            ->defaultSort('created_at', 'desc')
+            ->paginated([25, 50]);
+    }
+}