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)

services/ActivityLogService.ts

@@ -0,0 +1,35 @@
+import { ActivityLogRepository } from '@/repositories/ActivityLogRepository';
+import type { ActivityLog } from '@/lib/types';
+import type { Paginated } from '@/repositories/NotificationRepository';
+
+export interface LogActivityInput {
+  projectId: number;
+  actorId: number;
+  action: string;
+  targetType: string;
+  targetId: number | null;
+  metadata?: Record<string, unknown> | null;
+}
+
+/**
+ * アクティビティログ記録を担うService。
+ * 変更操作をプロジェクト単位の時系列ログとして記録する。
+ */
+export class ActivityLogService {
+  constructor(private readonly activityLogRepository: ActivityLogRepository) {}
+
+  logActivity(input: LogActivityInput): ActivityLog {
+    return this.activityLogRepository.create({
+      projectId: input.projectId,
+      actorId: input.actorId,
+      action: input.action,
+      targetType: input.targetType,
+      targetId: input.targetId,
+      metadataJson: input.metadata ? JSON.stringify(input.metadata) : null,
+    });
+  }
+
+  listByProject(projectId: number, page: number = 1): Paginated<ActivityLog> {
+    return this.activityLogRepository.findByProject(projectId, page);
+  }
+}