feat: Complete v1.1.1 Phases 2 & 3 - Enhancements and Polish

Phase 2 - Enhancements:
- Add performance monitoring infrastructure with MetricsBuffer
- Implement three-tier health checks (/health, /health?detailed, /admin/health)
- Enhance search with FTS5 fallback and XSS-safe highlighting
- Add Unicode slug generation with timestamp fallback
- Expose database pool statistics via /admin/metrics
- Create missing error templates (400, 401, 403, 405, 503)

Phase 3 - Polish:
- Implement RSS streaming optimization (memory O(n) → O(1))
- Add admin metrics dashboard with htmx and Chart.js
- Fix flaky migration race condition tests
- Create comprehensive operational documentation
- Add upgrade guide and troubleshooting guide

Testing: 632 tests passing, zero flaky tests
Documentation: Complete operational guides
Security: All security reviews passed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

templates/admin/base.html

@@ -5,6 +5,7 @@
   <nav class="admin-nav">
     <a href="{{ url_for('admin.dashboard') }}">Dashboard</a>
     <a href="{{ url_for('admin.new_note_form') }}">New Note</a>
+    <a href="{{ url_for('admin.metrics_dashboard') }}">Metrics</a>
     <form action="{{ url_for('auth.logout') }}" method="POST" class="logout-form">
       <button type="submit" class="button button-secondary">Logout</button>
     </form>