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)

components/layout/ProjectNav.tsx

@@ -2,6 +2,7 @@ const NAV_ITEMS = [
   { href: '', label: '概要' },
   { href: '/board', label: '掲示板' },
   { href: '/notes', label: 'メモ' },
+  { href: '/chat', label: 'チャット' },
   { href: '/members', label: 'メンバー' },
   { href: '/activity', label: 'アクティビティ' },
   { href: '/settings', label: '設定' },
@@ -16,12 +17,20 @@ export function ProjectNav({
   active,
 }: {
   projectId: number;
-  active: 'overview' | 'board' | 'notes' | 'members' | 'activity' | 'settings';
+  active:
+    | 'overview'
+    | 'board'
+    | 'notes'
+    | 'chat'
+    | 'members'
+    | 'activity'
+    | 'settings';
 }) {
   const activeMap: Record<string, boolean> = {
     overview: active === 'overview',
     board: active === 'board',
     notes: active === 'notes',
+    chat: active === 'chat',
     members: active === 'members',
     activity: active === 'activity',
     settings: active === 'settings',