feat(m5): notifications & activity log foundation

- Expand NotificationRepository (unread list paginated, count, markRead
  ownership-scoped); new ActivityLogRepository (create, findByProject
  paginated) with deterministic created_at+id ordering
- NotificationService (notifyOnEvent, resolveTargets for 8 event types,
  injectable SseBroadcaster for M8) + ActivityLogService (logActivity,
  listByProject)
- NotificationEvent/SseEvent/SseBroadcaster types in lib/types
- APIs: GET /api/notifications, POST /api/notifications/:id/read,
  GET /api/projects/:projectId/activity
- Screens: notifications list + MarkReadButton, NotificationBadge in
  Header, project activity page + ProjectNav link
- Unit tests for both repositories and services (28 new)

components/layout/ProjectNav.tsx

@@ -1,6 +1,7 @@
 const NAV_ITEMS = [
   { href: '', label: '概要' },
   { href: '/members', label: 'メンバー' },
+  { href: '/activity', label: 'アクティビティ' },
   { href: '/settings', label: '設定' },
 ] as const;
 
@@ -13,11 +14,12 @@ export function ProjectNav({
   active,
 }: {
   projectId: number;
-  active: 'overview' | 'members' | 'settings';
+  active: 'overview' | 'members' | 'activity' | 'settings';
 }) {
   const activeMap: Record<string, boolean> = {
     overview: active === 'overview',
     members: active === 'members',
+    activity: active === 'activity',
     settings: active === 'settings',
   };