feat: Implement Siti Stock Plugin for WooCommerce integration

- Added core plugin structure with main class Siti_Stock_Plugin.
- Implemented settings management through Siti_Stock_Settings.
- Developed admin interface for settings configuration via Siti_Stock_Admin.
- Created inventory management with external stock handling in Siti_Stock_Inventory_Manager.
- Integrated synchronization service to fetch and apply stock updates from external API in Siti_Stock_Sync_Service.
- Added custom product data store to manage combined stock values in Siti_Stock_Product_Data_Store.
- Registered hooks for admin menus, settings, and synchronization processes.
- Implemented REST API endpoint for triggering stock sync.
- Added cron scheduling for automatic stock synchronization.
- Included localization support for Dutch language.

includes/class-siti-stock-product-data-store.php

@@ -0,0 +1,51 @@
+<?php
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Custom product data store that exposes combined (local + external) stock in SQL queries.
+ *
+ * WooCommerce's stock reservation system queries the raw `_stock` meta directly and therefore
+ * ignores any runtime filters. By decorating the default CPT data store we make sure that the
+ * reservation queries see the same combined stock value that the storefront shows.
+ */
+class Siti_Stock_Product_Data_Store extends WC_Product_Data_Store_CPT {
+
+	/**
+	 * Build a SQL snippet that adds the external stock meta value to the native `_stock`.
+	 *
+	 * {@inheritDoc}
+	 *
+	 * @param int $product_id Product ID that the stock query should represent.
+	 * @return string
+	 */
+	public function get_query_for_stock( $product_id ) {
+		global $wpdb;
+
+		return $wpdb->prepare(
+			"
+			SELECT
+				GREATEST(
+					0,
+					CAST( COALESCE( stock_meta.meta_value, 0 ) AS SIGNED )
+				) +
+				GREATEST(
+					0,
+					CAST( COALESCE( external_meta.meta_value, 0 ) AS SIGNED )
+				)
+			FROM {$wpdb->posts} AS posts
+			LEFT JOIN {$wpdb->postmeta} AS stock_meta
+				ON stock_meta.post_id = posts.ID AND stock_meta.meta_key = %s
+			LEFT JOIN {$wpdb->postmeta} AS external_meta
+				ON external_meta.post_id = posts.ID AND external_meta.meta_key = %s
+			WHERE posts.ID = %d
+			LIMIT 1
+			",
+			'_stock',
+			Siti_Stock_Plugin::EXTERNAL_STOCK_META_KEY,
+			$product_id
+		);
+	}
+}