Migrate user/profile URLs to /u/[handle] and remove legacy scripts

All user and post profile URLs have been updated from /[handle] to /u/[handle] for consistency. Added a redirect page for /posts/[id] to the canonical /u/[handle]/posts/[id] route. Deprecated and removed legacy scripts and documentation related to chat and federation endpoints. Push-based federation endpoints are now disabled in favor of real-time pull-based federation.

src/app/posts/[id]/page.tsx

@@ -0,0 +1,40 @@
+'use client';
+
+import { useEffect } from 'react';
+import { useParams, useRouter } from 'next/navigation';
+
+export default function PostRedirect() {
+    const params = useParams();
+    const router = useRouter();
+    const id = params.id as string;
+
+    useEffect(() => {
+        const fetchAndRedirect = async () => {
+            try {
+                const res = await fetch(`/api/posts/${id}`);
+                if (!res.ok) {
+                    router.push('/');
+                    return;
+                }
+                const data = await res.json();
+                router.push(`/u/${data.post.author.handle}/posts/${id}`);
+            } catch {
+                router.push('/');
+            }
+        };
+
+        fetchAndRedirect();
+    }, [id, router]);
+
+    return (
+        <div style={{
+            minHeight: '100vh',
+            display: 'flex',
+            alignItems: 'center',
+            justifyContent: 'center',
+            color: 'var(--foreground-tertiary)',
+        }}>
+            Loading...
+        </div>
+    );
+}