Add book catalog example with global/local template system

Features:
- Global template configuration (sqlite_global_templates directive)
- Template path resolution relative to location
- Automatic template loading from directories
- Local templates override global templates
- Content handler installation via directive setter

Book Catalog Example:
- Complete working example with 10 technical books
- SQLite database with setup script
- 4 browseable pages (all, programming, databases, computer-science)
- Shared global templates (header, footer, book_card partial)
- Category-specific local templates with unique theming
- Responsive gradient UI design
- Working navigation and filtering

Configuration:
- sqlite_global_templates: HTTP main-level directive for shared templates
- sqlite_template: Location-level directive (sets content handler)
- Template resolution: {doc_root}{uri}/{template_name}
- All .hbs files in directories auto-loaded as partials

Technical improvements:
- Fixed content handler setup (not phase handler)
- Proper HttpModuleMainConf and HttpModuleLocationConf traits
- Template directory scanning and registration
- Error handling with debug logging

server_root/global_templates/footer.hbs

@@ -0,0 +1,9 @@
+        </main>
+        <footer style="background: #f8f9fa; padding: 2rem; text-align: center; border-top: 2px solid #e9ecef; color: #6c757d;">
+            <p>📖 Powered by nginx-test SQLite Module</p>
+            <p style="margin-top: 0.5rem; font-size: 0.9rem;">A demonstration of Rust + NGINX + SQLite + Handlebars</p>
+        </footer>
+    </div>
+</body>
+</html>
+