Add Webhook Endpoints for external automation

- Create WebhookController with 4 endpoints:
  - POST /api/webhooks/posts - Create a new post
  - GET /api/webhooks/posts - List posts with filters
  - POST /api/webhooks/posts/{post}/publish - Trigger publishing
  - GET /api/webhooks/posts/{post}/status - Get post status
- Add X-Muninn-Token header authentication
- Add MUNINN_WEBHOOK_TOKEN config in services.php
- Add MUNINN_WEBHOOK_TOKEN to .env.example
- Add webhook routes with /webhooks prefix

All endpoints return JSON responses with success/error fields.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

.env.example

@@ -62,4 +62,7 @@ AWS_DEFAULT_REGION=us-east-1
 AWS_BUCKET=
 AWS_USE_PATH_STYLE_ENDPOINT=false
 
+# Muninn webhook authentication token (required for webhook endpoints)
+MUNINN_WEBHOOK_TOKEN=
+
 VITE_APP_NAME="${APP_NAME}"