feat(m8): SSE hub + realtime chat with mentions, tests, e2e

- SseHub (project-scoped client mgmt + broadcast, implements SseBroadcaster)
  with SSE-formatted payload; singleton getSseHub
- ChatRepository (message CRUD, soft-delete, search, pagination, project
  isolation) + ChatService (membership permission, send/edit/delete,
  @email mention -> mention notification, SSE broadcast of created/updated/deleted)
- SSE stream endpoint (ReadableStream + heartbeat + abort cleanup, membership)
- Chat message API routes (history/send/edit/delete)
- ChatWindow client (EventSource auto-reconnect, history fetch, realtime
  append/update/delete), chat page, ProjectNav チャット link
- SseEvent type now carries ChatMessage for chat events
- Unit (SseHub, ChatRepository, ChatService) + integration chat-sse-broadcast
  + e2e chat-sse (two contexts realtime)

app/projects/[projectId]/chat/page.tsx

@@ -0,0 +1,41 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import { createProjectService } from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { ProjectNav } from '@/components/layout/ProjectNav';
+import { ChatWindow } from '@/components/chat/ChatWindow';
+import { ForbiddenError, NotFoundError } from '@/lib/errors';
+
+export const dynamic = 'force-dynamic';
+
+export default async function ChatPage({
+  params,
+}: {
+  params: Promise<{ projectId: string }>;
+}) {
+  const user = await getCurrentUser();
+  if (!user) redirect('/login');
+  const { projectId } = await params;
+
+  const projectService = createProjectService();
+  let project: Awaited<ReturnType<typeof projectService.getProject>>;
+  try {
+    project = projectService.getProject(user.id, Number(projectId));
+  } catch (error) {
+    if (error instanceof ForbiddenError || error instanceof NotFoundError) {
+      redirect('/dashboard');
+    }
+    throw error;
+  }
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <ProjectNav projectId={project.id} active="chat" />
+      <main className="mx-auto max-w-3xl space-y-4 p-6">
+        <h1 className="text-2xl font-bold">チャット</h1>
+        <ChatWindow projectId={project.id} userName={user.name} />
+      </main>
+    </div>
+  );
+}