feat(admin): add ephemeral staging DB, read-only posts overview, and dynamic search

- Implement ephemeral staging mode (APP_ENV=staging) to isolate test/dev DB writes in /tmp
- Add read-only posts management dashboard under /guanli/posts with live links and comment counters
- Add hot re-indexing endpoint (/guanli/reindex) to refresh Markdown without restarting server
- Replace static recommended search config with dynamic frequency-based trending searches
- Remove early scaffold mockup file (dropbox/zh.md) and clean legacy mock test queries
- Add native Makefile with dev, dev-persistent, test, lint, and docker targets
- Update test suite and documentation for development workflows

views/partials/admin-header.ejs

@@ -10,6 +10,10 @@
 
     <!-- Admin Navigation Tabs -->
     <div style="display: flex; gap: 0.5rem; align-items: center; flex-wrap: wrap;">
+      <a href="/guanli/posts" class="btn <%= activeAdminTab === 'posts' ? 'btn-primary' : 'btn-secondary' %>" style="padding: 0.45rem 0.95rem; font-size: 0.88rem;">
+        📝 Posts
+      </a>
+
       <a href="/guanli/comments" class="btn <%= activeAdminTab === 'comments' ? 'btn-primary' : 'btn-secondary' %>" style="padding: 0.45rem 0.95rem; font-size: 0.88rem;">
         💬 Comments
         <% if (stats && stats.pendingComments > 0) { %>