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/notifications/NotificationBadge.tsx

@@ -0,0 +1,43 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+
+export function NotificationBadge() {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    let active = true;
+    fetch('/api/notifications?page=1')
+      .then((res) => (res.ok ? res.json() : null))
+      .then((data) => {
+        if (active && data && typeof data.total === 'number') {
+          setCount(data.total);
+        }
+      })
+      .catch(() => undefined);
+    return () => {
+      active = false;
+    };
+  }, []);
+
+  if (count === 0) {
+    return (
+      <a href="/notifications" className="text-gray-600 hover:underline">
+        通知
+      </a>
+    );
+  }
+
+  return (
+    <a
+      href="/notifications"
+      className="relative text-gray-600 hover:underline"
+      data-notification-count={count}
+    >
+      通知
+      <span className="ml-1 rounded-full bg-red-500 px-1.5 py-0.5 text-xs text-white">
+        {count > 99 ? '99+' : count}
+      </span>
+    </a>
+  );
+}