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/api/projects/[projectId]/chat/stream/route.ts

@@ -0,0 +1,70 @@
+import { NextRequest } from 'next/server';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+import { createProjectService } from '@/lib/api/services';
+import { getSseHub } from '@/lib/sse/hub';
+import { UnauthorizedError } from '@/lib/errors';
+import { handleApiError } from '@/lib/api/handleError';
+
+export const runtime = 'nodejs';
+
+export async function GET(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { projectId } = await params;
+
+  const projectService = createProjectService();
+  try {
+    projectService.getProject(user.id, Number(projectId));
+  } catch (error) {
+    return handleApiError(error);
+  }
+
+  const pid = Number(projectId);
+  const hub = getSseHub();
+  const encoder = new TextEncoder();
+
+  const stream = new ReadableStream<Uint8Array>({
+    start(controller) {
+      const client = {
+        enqueue: (chunk: string) => controller.enqueue(encoder.encode(chunk)),
+        close: () => {
+          try {
+            controller.close();
+          } catch {
+            // 既に閉じられている
+          }
+        },
+      };
+      hub.addClient(pid, client);
+
+      const heartbeat = setInterval(() => {
+        try {
+          controller.enqueue(encoder.encode(': ping\n\n'));
+        } catch {
+          clearInterval(heartbeat);
+        }
+      }, 30_000);
+
+      request.signal.addEventListener('abort', () => {
+        clearInterval(heartbeat);
+        hub.removeClient(pid, client);
+        try {
+          controller.close();
+        } catch {
+          // 既に閉じられている
+        }
+      });
+    },
+  });
+
+  return new Response(stream, {
+    headers: {
+      'Content-Type': 'text/event-stream',
+      'Cache-Control': 'no-cache, no-transform',
+      Connection: 'keep-alive',
+    },
+  });
+}