fix: restructure inbox API routes — channel/ and message/ namespaces

Next.js не позволяет [channelId] и [id] на одном уровне.
Решение: /api/inbox/channel/[channelId]/* и /api/inbox/message/[id]/*
InboxTab: все fetch пути обновлены

app/api/inbox/channel/[channelId]/route.js

@@ -0,0 +1,16 @@
+import { NextResponse } from 'next/server';
+import { requireUser } from '@/lib/session';
+
+const ENGINE_URL    = process.env.ENGINE_URL    || 'http://localhost:3030';
+const ENGINE_SECRET = process.env.ENGINE_SECRET || '';
+
+export async function GET(req, { params }) {
+  const user = await requireUser();
+  if (!user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  const { searchParams } = new URL(req.url);
+  const res = await fetch(
+    `${ENGINE_URL}/api/inbox/${params.channelId}?${searchParams.toString()}`,
+    { headers: { 'x-internal-secret': ENGINE_SECRET, 'x-user-id': String(user.id) } }
+  );
+  return NextResponse.json(await res.json());
+}