feat: Phase 1 - Backend API meta compatibility (Level 1)

**Implemented: Backend API Enhancement for Level 1 Compatibility**

Following IMPLEMENTATION_PLAN_META_COMPAT.md Phase 1

**OrdersController.php:**
✅ Added get_order_meta_data() - Expose meta in API responses
✅ Added update_order_meta_data() - Update meta from API
✅ Modified show() - Include meta in response
✅ Modified update() - Handle meta updates
✅ Added filter: woonoow/order_allowed_private_meta
✅ Added filter: woonoow/order_updatable_meta
✅ Added filter: woonoow/order_api_data
✅ Added action: woonoow/order_updated

**ProductsController.php:**
✅ Added get_product_meta_data() - Expose meta in API responses
✅ Added update_product_meta_data() - Update meta from API
✅ Modified format_product_full() - Include meta in response
✅ Modified update_product() - Handle meta updates
✅ Added filter: woonoow/product_allowed_private_meta
✅ Added filter: woonoow/product_updatable_meta
✅ Added filter: woonoow/product_api_data
✅ Added action: woonoow/product_updated

**Meta Filtering Logic:**
- Skip internal WooCommerce meta (_wc_*)
- Skip WooNooW internal meta (_woonoow_*)
- Public meta (no underscore) - always expose
- Private meta (starts with _) - check allowed list
- Plugins can add to allowed list via filters

**Default Allowed Meta (Orders):**
- _tracking_number
- _tracking_provider
- _tracking_url
- _shipment_tracking_items
- _wc_shipment_tracking_items
- _transaction_id
- _payment_method_title

**How It Works:**
1. Plugin stores: update_post_meta($order_id, '_tracking_number', '123')
2. WooNooW API exposes: GET /orders/123 returns meta._tracking_number
3. Frontend can read/write via API
4. Plugin works WITHOUT any extra effort

**Next Steps:**
- Phase 2: Frontend components (MetaFields, useMetaFields)
- Phase 3: PHP MetaFieldsRegistry system
- Testing with popular plugins

**Status:** Backend API ready for Level 1 compatibility! 🎉

includes/Api/ProductsController.php

@@ -371,8 +371,16 @@ class ProductsController {
 			$product->set_gallery_image_ids($data['gallery_image_ids']);
 		}
 		
+		// Update custom meta fields (Level 1 compatibility)
+		if (isset($data['meta']) && is_array($data['meta'])) {
+			self::update_product_meta_data($product, $data['meta']);
+		}
+		
 		$product->save();
 		
+		// Allow plugins to perform additional updates (Level 1 compatibility)
+		do_action('woonoow/product_updated', $product, $data, $request);
+		
 		// Handle variations for variable products
 		if ($product->is_type('variable')) {
 			if (isset($data['attributes'])) {
@@ -565,6 +573,12 @@ class ProductsController {
 			$data['variations'] = self::get_product_variations($product);
 		}
 		
+		// Expose meta data (Level 1 compatibility)
+		$data['meta'] = self::get_product_meta_data($product);
+		
+		// Allow plugins to modify response (Level 1 compatibility)
+		$data = apply_filters('woonoow/product_api_data', $data, $product);
+		
 		return $data;
 	}
 	
@@ -697,4 +711,89 @@ class ProductsController {
 			}
 		}
 	}
+	
+	/**
+	 * Get product meta data for API exposure (Level 1 compatibility)
+	 * Filters out internal meta unless explicitly allowed
+	 * 
+	 * @param \WC_Product $product
+	 * @return array
+	 */
+	private static function get_product_meta_data($product) {
+		$meta_data = [];
+		
+		foreach ($product->get_meta_data() as $meta) {
+			$key = $meta->key;
+			$value = $meta->value;
+			
+			// Skip internal WooCommerce meta (starts with _wc_)
+			if (strpos($key, '_wc_') === 0) {
+				continue;
+			}
+			
+			// Skip WooNooW internal meta
+			if (strpos($key, '_woonoow_') === 0) {
+				continue;
+			}
+			
+			// Public meta (no underscore) - always expose
+			if (strpos($key, '_') !== 0) {
+				$meta_data[$key] = $value;
+				continue;
+			}
+			
+			// Private meta (starts with _) - check if allowed
+			$allowed_private = apply_filters('woonoow/product_allowed_private_meta', [
+				// Common custom fields
+				'_custom_field',
+				
+				// Allow plugins to add their meta via filter
+			], $product);
+			
+			if (in_array($key, $allowed_private, true)) {
+				$meta_data[$key] = $value;
+			}
+		}
+		
+		return $meta_data;
+	}
+	
+	/**
+	 * Update product meta data from API (Level 1 compatibility)
+	 * 
+	 * @param \WC_Product $product
+	 * @param array $meta_updates
+	 */
+	private static function update_product_meta_data($product, $meta_updates) {
+		// Get allowed updatable meta keys
+		$allowed = apply_filters('woonoow/product_updatable_meta', [
+			// Common custom fields
+			'_custom_field',
+			
+			// Allow plugins to add their meta via filter
+		], $product);
+		
+		foreach ($meta_updates as $key => $value) {
+			// Skip internal WooCommerce meta
+			if (strpos($key, '_wc_') === 0) {
+				continue;
+			}
+			
+			// Skip WooNooW internal meta
+			if (strpos($key, '_woonoow_') === 0) {
+				continue;
+			}
+			
+			// Public meta (no underscore) - always allow
+			if (strpos($key, '_') !== 0) {
+				$product->update_meta_data($key, $value);
+				continue;
+			}
+			
+			// Private meta - check if allowed
+			if (in_array($key, $allowed, true)) {
+				$product->update_meta_data($key, $value);
+			}
+		}
+	}
 }