Restaurant mode Phase 2: menu depth — categories, stations, modifiers, courses

Builds on Phase 1 with the menu structure restaurants need:
- Menu setup page (restaurant mode): categories, kitchen stations, and modifier
  groups with options (price deltas).
- Products gain category, kitchen station, default course, and attached modifier
  groups (managed on the product form).
- Ordering: the ticket groups the catalog by category and, when a product has
  modifier groups, opens a picker (respects min/max select) for options + notes +
  course before adding. Effective price = base + modifier deltas (resolved
  server-side; client prices are never trusted).
- Fire-by-course: send the whole tab or just one course to the kitchen.
- KDS: filter by station; each item shows its station, course, modifiers and notes.

Schema additive (pos_categories, pos_stations, pos_modifier_groups, pos_modifiers,
product↔group pivot, pos_sale_line_modifiers; category/station/course columns on
products and lines). PosRestaurantTest covers modifiers, course and station routing;
suite green (11 passed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Models/PosSaleLineModifier.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class PosSaleLineModifier extends Model
+{
+    protected $fillable = ['pos_sale_line_id', 'modifier_id', 'name', 'price_delta_minor'];
+
+    protected function casts(): array
+    {
+        return ['price_delta_minor' => 'integer'];
+    }
+
+    public function line(): BelongsTo
+    {
+        return $this->belongsTo(PosSaleLine::class, 'pos_sale_line_id');
+    }
+}