Add queue timeout voicemail and Amazon SNS SMS provider support

This update adds two major features:

1. Queue Timeout Voicemail
   - Callers can now leave voicemail when queue timeout is reached
   - Configurable per-queue voicemail prompts with TTS support
   - Automatic transcription and urgent keyword detection
   - Admin setting to choose between voicemail or callback on timeout

2. Amazon SNS SMS Provider
   - Alternative SMS provider to Twilio for sending text messages
   - Useful when Twilio SMS approval is difficult to obtain
   - Provider abstraction layer allows switching between Twilio/SNS
   - Full AWS SNS configuration in admin settings
   - Supports custom sender IDs in compatible countries
   - Lower cost per SMS compared to Twilio

New Files:
- includes/class-twp-voicemail-handler.php - Voicemail recording handler
- includes/interface-twp-sms-provider.php - SMS provider interface
- includes/class-twp-sms-provider-twilio.php - Twilio SMS implementation
- includes/class-twp-sms-provider-sns.php - Amazon SNS implementation
- includes/class-twp-sms-manager.php - SMS provider abstraction manager
- QUEUE_VOICEMAIL_SMS_FEATURES.md - Complete feature documentation

Modified Files:
- includes/class-twp-call-queue.php - Added voicemail option to timeout handler
- includes/class-twp-twilio-api.php - Updated send_sms() to use provider abstraction
- admin/class-twp-admin.php - Added SMS provider and timeout action settings
- composer.json - Added AWS SDK dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

composer.json

@@ -4,7 +4,11 @@
     "type": "wordpress-plugin",
     "require": {
         "php": ">=8.0",
-        "twilio/sdk": "^8.7"
+        "twilio/sdk": "^8.7",
+        "aws/aws-sdk-php": "^3.0"
+    },
+    "suggest": {
+        "aws/aws-sdk-php": "Required for Amazon SNS SMS provider support"
     },
     "autoload": {
         "classmap": [