feat(threads): add SEO metadata, sitemap, and structured data

Include /threads in sitemap and set canonical paths
Generate DiscussionForumPosting structured data for thread details
Add dynamic SEO updates for thread navigation and server-side rendering

frontend/pages/threads/index.vue

@@ -2,7 +2,8 @@
 import ThreadsView from '../../src/views/ThreadsView.vue';
 
 definePageMeta({
-  title: 'Threads'
+  name: 'threads',
+  seo: { titleKey: 'pages.threads.title', descriptionKey: 'seo.threadsDescription', canonicalPath: '/threads' }
 });
 </script>