feat: add DatabaseDriver

Implements CartDriver for Eloquent-backed persistence using firstOrNew
pattern. Includes 9 unit tests covering CRUD, flush, clearItems, and
model retrieval.

tests/Unit/Drivers/DatabaseDriverTest.php

@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+use Wearepixel\Cart\Drivers\DatabaseDriver;
+use Wearepixel\Cart\Tests\Helpers\MockCartModel;
+
+beforeEach(function () {
+    $this->driver = new DatabaseDriver(
+        MockCartModel::class,
+        'session_id',
+        'items',
+        'conditions',
+        'test-session',
+    );
+});
+
+describe('DatabaseDriver', function () {
+    test('returns its session key', function () {
+        expect($this->driver->getSessionKey())->toBe('test-session');
+    });
+
+    test('returns database as driver name', function () {
+        expect($this->driver->getDriverName())->toBe('database');
+    });
+
+    test('starts with empty items', function () {
+        expect($this->driver->getItems())->toBe([]);
+    });
+
+    test('stores and retrieves items', function () {
+        $items = [['id' => 1, 'name' => 'Widget']];
+        $this->driver->putItems($items);
+
+        $fresh = new DatabaseDriver(MockCartModel::class, 'session_id', 'items', 'conditions', 'test-session');
+        expect($fresh->getItems())->toBe($items);
+    });
+
+    test('starts with empty conditions', function () {
+        expect($this->driver->getConditions())->toBe([]);
+    });
+
+    test('stores and retrieves conditions', function () {
+        $conditions = [['name' => 'GST', 'type' => 'tax', 'value' => '10%']];
+        $this->driver->putConditions($conditions);
+
+        $fresh = new DatabaseDriver(MockCartModel::class, 'session_id', 'items', 'conditions', 'test-session');
+        expect($fresh->getConditions())->toBe($conditions);
+    });
+
+    test('flush deletes the database record', function () {
+        $this->driver->putItems([['id' => 1]]);
+        $this->driver->flush();
+
+        expect(MockCartModel::count())->toBe(0);
+    });
+
+    test('clearItems empties items but keeps conditions', function () {
+        $this->driver->putItems([['id' => 1]]);
+        $this->driver->putConditions([['name' => 'GST']]);
+        $this->driver->clearItems();
+
+        $fresh = new DatabaseDriver(MockCartModel::class, 'session_id', 'items', 'conditions', 'test-session');
+        expect($fresh->getItems())->toBe([]);
+        expect($fresh->getConditions())->toBe([['name' => 'GST']]);
+    });
+
+    test('getSessionModel returns the Eloquent model', function () {
+        expect($this->driver->getSessionModel())->toBeInstanceOf(MockCartModel::class);
+    });
+});