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);
+  }
+}

services/NotificationService.ts

@@ -0,0 +1,84 @@
+import { NotificationRepository } from '@/repositories/NotificationRepository';
+import type { SseBroadcaster } from '@/lib/types';
+import type {
+  Notification,
+  NotificationEvent,
+  NotificationType,
+} from '@/lib/types';
+import type { Paginated } from '@/repositories/NotificationRepository';
+
+/**
+ * 通知生成を担うService。
+ * イベント種別に応じた対象ユーザー解決(resolveTargets)と通知作成を行う。
+ * SSE配信は注入可能なbroadcaster経由(M8でSseHubを注入。M5では省略可能)。
+ */
+export class NotificationService {
+  constructor(
+    private readonly notificationRepository: NotificationRepository,
+    private readonly broadcaster?: SseBroadcaster | null
+  ) {}
+
+  /** イベント発生時に通知を作成し、SSE配信する */
+  notifyOnEvent(event: NotificationEvent): Notification[] {
+    const targetUserIds = this.resolveTargets(event);
+    const created: Notification[] = [];
+    for (const userId of targetUserIds) {
+      const notification = this.notificationRepository.create({
+        userId,
+        projectId: event.projectId,
+        type: event.type,
+        title: event.title,
+        body: event.body,
+      });
+      created.push(notification);
+    }
+    if (event.projectId !== null && this.broadcaster) {
+      this.broadcaster.broadcast(event.projectId, {
+        type: 'notification.created',
+        data: { projectId: event.projectId },
+      });
+    }
+    return created;
+  }
+
+  /** イベント種別から対象ユーザーIDの集合を解決する */
+  resolveTargets(event: NotificationEvent): number[] {
+    switch (event.type) {
+      case 'mention':
+        return [event.mentionedUserId];
+      case 'todo_assigned':
+        return [event.assigneeId];
+      case 'todo_due_soon':
+        return [event.assigneeId];
+      case 'meeting_invited':
+        return event.memberIds;
+      case 'board_commented':
+        return [event.threadAuthorId];
+      case 'project_added':
+        return [event.addedUserId];
+      case 'file_shared':
+        return event.projectMemberIds;
+      case 'note_updated':
+        return event.projectMemberIds;
+    }
+  }
+
+  listUnread(userId: number, page: number = 1): Paginated<Notification> {
+    return this.notificationRepository.findUnreadByUser(userId, page);
+  }
+
+  countUnread(userId: number): number {
+    return this.notificationRepository.countUnreadByUser(userId);
+  }
+
+  markRead(id: number, userId: number): boolean {
+    return this.notificationRepository.markRead(id, userId);
+  }
+}
+
+/** 型安全性のためのヘルパー: NotificationEvent.type → NotificationType */
+export function eventTypeToType(
+  type: NotificationEvent['type']
+): NotificationType {
+  return type;
+}