Implement Phase 10: REST API Endpoints (v0.10.0)

- Add complete REST API infrastructure under src/Api/
- ResponseFormatter for standardized responses
- RateLimiter with tiered limits (public 60/min, availability 30/min, booking 10/min, admin 120/min)
- AbstractController base class with common functionality
- BuildingsController: list, get, rooms endpoints
- RoomsController: list, get, availability, calendar, search endpoints
- BookingsController: CRUD + confirm/check-in/check-out status transitions
- GuestsController: list, get, search, booking history (admin only)
- ServicesController: list, get, calculate endpoints
- PricingController: calculate, seasons endpoints
- API settings tab with enable/disable toggles
- Comprehensive API documentation in README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wp-bnb.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP BnB Management
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-bnb
  * Description: A comprehensive Bed & Breakfast management system for WordPress. Manage buildings, rooms, bookings, and guests.
- * Version: 0.9.0
+ * Version: 0.10.0
  * Requires at least: 6.0
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -24,7 +24,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 }
 
 // Plugin version constant - MUST match Version in header above.
-define( 'WP_BNB_VERSION', '0.9.0' );
+define( 'WP_BNB_VERSION', '0.10.0' );
 
 // Plugin path constants.
 define( 'WP_BNB_PATH', plugin_dir_path( __FILE__ ) );