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/405.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+
+{% block title %}Method Not Allowed - {{ config.SITE_NAME }}{% endblock %}
+
+{% block content %}
+<article class="error-page">
+  <h1>405 - Method Not Allowed</h1>
+  <p>Sorry, the HTTP method you used is not allowed for this resource.</p>
+  <p><a href="/">Return to homepage</a></p>
+</article>
+{% endblock %}