feat(api): expose last_wash timestamp on /numberplatescans (TRU-78) (#401)

Resolves TRU-78 (DRIFT 17: License plate scan - show 'last washed'
timestamp on landing page / DHL use case).

The POS landing page already surfaces license plate scans via `GET
/numberplatescans`, but it has no way to tell the operator **when a
plate was last washed**. With DHL trailers going in and out several
times a day, the front-desk needs that hint to decide whether a trailer
needs another wash before pick-up.

## Changes

- **`orders_o::getLastWashTimestampForPlate(string $reg_1): ?string`** —
new helper that returns the `created_at` (MySQL DATETIME) of the most
recent non-deleted order for the plate that has at least one non-deleted
order item. Mirrors the contract used by
`customer_vehicles_o::getLastOrderByPlate()` so the timestamp is always
backed by a real wash.
- **`GET /numberplatescans`** now enriches each scan row with a
`last_wash` key (string or `null`). No breaking change to the existing
payload; new field is additive.
- **New Pest test**
`services/nginx/app/tests/Unit/Orders/OrderLastWashTimestampForPlateTest.php`
— static-analysis assertions for the helper definition and the route
wiring (matches the style of `OrderBookingsCountsRouteWiringTest`).

## Frontend companion

https://github.com/copenhagentruckwash/pleno-vue/pull/335 renders this
`last_wash` in the inline details of each scan row on the POS landing
page (`PosLastScannedLicensePlatesV2.vue`), with an "Aldrig vasket" /
"Never washed" fallback when the API returns `null`.

## Risk

- `getLastWashTimestampForPlate` does one extra indexed read per scan
row (`SELECT id FROM orders WHERE reg_1 = ? AND deleted_at IS NULL`).
The existing `isPlateSeenBefore` call already does the same, so the
route's per-row query count is unchanged in shape.
- The new field is additive and ignored by older clients, so this can
roll forward without a coordinated client release.

Co-authored-by: openclaw bugfix <openclaw@copenhagentruckwash.local>

services/nginx/app/routes/plateScansRoute.php

@@ -120,11 +120,21 @@ class plateScansRoute
                                 'type' => (int)$tmp_scan_vehicle['type'],
                             ];
                         }
+                        // TRU-78 / DRIFT 17: enrich each scan with the
+                        // timestamp of the most recent completed wash for
+                        // that plate so the POS landing page can show
+                        // "last washed" at a glance when DHL trailers are
+                        // being picked up.
+                        $plate_value = (string)$scan['plate'];
+                        $tmp_scan_last_wash = [
+                            'last_wash' => (new orders_o())->getLastWashTimestampForPlate($plate_value),
+                        ];
                         // Return the object as an array
                         return [
                             ...$scan,
                             ...$tmp_scan_customer,
                             ...$tmp_scan_seen_before,
+                            ...$tmp_scan_last_wash,
                         ];
                     },
                     $number_plate_scans->forceRestrictFilters(