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)

repositories/ActivityLogRepository.ts

@@ -0,0 +1,93 @@
+import type { SqliteDatabase } from '@/lib/db/sqlite';
+import type { ActivityLog } from '@/lib/types';
+import type { Paginated } from '@/repositories/NotificationRepository';
+
+const DEFAULT_PAGE_SIZE = 20;
+
+export interface CreateActivityLogInput {
+  projectId: number;
+  actorId: number;
+  action: string;
+  targetType: string;
+  targetId: number | null;
+  metadataJson: string | null;
+}
+
+interface ActivityLogRow {
+  id: number;
+  project_id: number;
+  actor_id: number;
+  action: string;
+  target_type: string;
+  target_id: number | null;
+  metadata_json: string | null;
+  created_at: string;
+}
+
+function mapActivityLog(row: ActivityLogRow): ActivityLog {
+  return {
+    id: row.id,
+    projectId: row.project_id,
+    actorId: row.actor_id,
+    action: row.action,
+    targetType: row.target_type,
+    targetId: row.target_id,
+    metadataJson: row.metadata_json,
+    createdAt: row.created_at,
+  };
+}
+
+/**
+ * activity_logsテーブルへのデータアクセスを担うRepository。
+ */
+export class ActivityLogRepository {
+  constructor(private readonly db: SqliteDatabase) {}
+
+  create(input: CreateActivityLogInput): ActivityLog {
+    const now = new Date().toISOString();
+    const result = this.db.execute(
+      `INSERT INTO activity_logs (project_id, actor_id, action, target_type, target_id, metadata_json, created_at)
+       VALUES (@projectId, @actorId, @action, @targetType, @targetId, @metadataJson, @createdAt)`,
+      {
+        projectId: input.projectId,
+        actorId: input.actorId,
+        action: input.action,
+        targetType: input.targetType,
+        targetId: input.targetId,
+        metadataJson: input.metadataJson,
+        createdAt: now,
+      }
+    );
+    return {
+      id: Number(result.lastInsertRowid),
+      projectId: input.projectId,
+      actorId: input.actorId,
+      action: input.action,
+      targetType: input.targetType,
+      targetId: input.targetId,
+      metadataJson: input.metadataJson,
+      createdAt: now,
+    };
+  }
+
+  /** プロジェクト別のアクティビティログ一覧（作成日時降順・ページネーション） */
+  findByProject(
+    projectId: number,
+    page: number = 1,
+    pageSize: number = DEFAULT_PAGE_SIZE
+  ): Paginated<ActivityLog> {
+    const offset = (page - 1) * pageSize;
+    const items = this.db.query<ActivityLogRow>(
+      `SELECT * FROM activity_logs
+       WHERE project_id = @projectId
+       ORDER BY created_at DESC, id DESC
+       LIMIT @pageSize OFFSET @offset`,
+      { projectId, pageSize, offset }
+    );
+    const row = this.db.get<{ count: number }>(
+      'SELECT COUNT(*) AS count FROM activity_logs WHERE project_id = @projectId',
+      { projectId }
+    );
+    return { items: items.map(mapActivityLog), total: row?.count ?? 0 };
+  }
+}