## Why

Lunch items currently have no price. Employees cannot see how much a dish costs when ordering, and there is no way to total up how much each employee owes for the month, so leaders currently have no system-backed way to collect lunch money at month end.

## What Changes

- Add a `price` field to lunch items (`lunch_items.price`), nullable/not required, so admins can price dishes via the existing create/update item endpoints and via the "custom item" shortcut used when creating a menu.
- Existing lunch items are backfilled to the current uniform price of 32,000 VND.
- Snapshot the ordered item's price onto each `lunch_orders` row at the moment an order is created or its item is changed, so a price change later does not rewrite the cost of orders already placed.
- Existing orders are backfilled from their item's (now-priced) `price` so in-progress billing periods stay correct.
- Extend the admin monthly order report (`GET /admin/lunch-orders/report`) with a per-employee total amount, computed from the snapshotted order prices, so leaders can use it to collect money at month end.
- Surface `price` wherever item/order data is already returned to clients (item list/detail, menu detail, menu report, order export) so employees can see a dish's price before choosing it.

## Capabilities

### New Capabilities
- `lunch-item-pricing`: dishes carry a price, orders snapshot that price at order time, and admins can see a per-employee total amount over a date range for billing.

### Modified Capabilities
(none — lunch ordering has no existing spec in `openspec/specs/`; all behavior above is captured under the new `lunch-item-pricing` capability)

## Impact

- **Migrations**: new columns `lunch_items.price` (nullable decimal, backfilled to 32000) and `lunch_orders.price` (nullable decimal, backfilled from item price).
- **Models**: `LunchItem`, `LunchOrder` (fillable + cast).
- **Controllers**: `LunchItemController` (store/update validation), `LunchMenuController` (custom_items validation + creation), `LunchOrderController` (store/update/createByAdmin/updateByAdmin snapshot logic, ordersReport aggregation).
- **Export**: `OrderExport` (add price column).
- **Package**: `packages/w3suga/lunch-ordering` only; no other package or app-level table depends on these columns besides the existing `app/Http/Controllers/Api/V1/Admin/DashboardController.php` read of `LunchBookingMenu` summaries.