feat: add admin crawl, cleanup, delete, logs endpoints with scheduler and tests

- Add POST /admin/crawl with TaskLock-based reentrancy guard
- Add POST /admin/cleanup (tmp files older than 24h) with CrawlLog
- Add POST /admin/delete with date range and 'DELETE' confirm token
- Add GET /admin/logs (paginated CrawlLog + DataDeleteJob viewer)
- Add app/services/cleaner.py (cleanup_tmp, delete_papers_by_date_range)
- Add app/services/scheduler.py (APScheduler daily crawl/cleanup jobs)
- Wire scheduler startup/shutdown hooks in app/main.py
- Add admin nav link in base.html and APP_HOST security warning
- Add apscheduler>=3.10 dependency
- Add tests/test_admin_phase4.py covering the new endpoints

app/templates/base.html

@@ -17,6 +17,7 @@
         <a href="/day/{{ today if today else '' }}">今日</a>
         <a href="/search">搜索</a>
         <a href="/reading-list">阅读列表</a>
+        <a href="/admin/logs">管理</a>
       </div>
     </nav>
   </header>