docs: Add comprehensive backend handler documentation

Create public-facing documentation explaining backend architecture:

New Documentation:
- doc/14-BACKEND-HANDLERS.md (900+ lines)
  * Handler architecture and file organization
  * Request/response type system with examples
  * Middleware pattern and preferences handling
  * Comprehensive testing strategy
  * Data flow diagrams and best practices
  * Code examples for all major patterns

Updated:
- doc/README.md
  * Add Backend Handlers to technical implementation section
  * Update total active docs count (13 → 14)
  * Add quick navigation links

Content Coverage:
- Handler responsibilities (pages, PDF, HTMX)
- Type-safe request handling with validation
- Middleware architecture and context usage
- Test coverage across all handler types
- Request processing flow diagrams
- Best practices with do/don't examples

Audience:
- Backend developers
- API consumers
- New contributors
- Technical documentation readers

Complements:
- Educational docs in _go-learning/refactorings/
- Internal architecture documentation
- API reference guide

doc/README.md

@@ -18,6 +18,7 @@
 - [5. Zoom Implementation](5-ZOOM-IMPLEMENTATION.md) - Custom zoom feature technical details
 - [12. CSS Architecture](12-CSS-ARCHITECTURE.md) - Modular CSS structure and ITCSS organization ⭐
 - [13. Toast Notifications](13-TOAST-NOTIFICATIONS.md) - Toast notification system for PDF downloads and user feedback
+- [14. Backend Handlers](14-BACKEND-HANDLERS.md) - Handler architecture, type safety, middleware, and testing ⭐
 
 **Deployment & Operations**
 - [8. Deployment Guide](8-DEPLOYMENT.md) - Production deployment instructions
@@ -46,6 +47,7 @@
 | 5 | [ZOOM_IMPLEMENTATION.md](5-ZOOM-IMPLEMENTATION.md) | Zoom feature implementation details | Feature developers |
 | 12 | [CSS-ARCHITECTURE.md](12-CSS-ARCHITECTURE.md) | Modular CSS structure, ITCSS layers, HTMX integration | Frontend developers, designers |
 | 13 | [TOAST-NOTIFICATIONS.md](13-TOAST-NOTIFICATIONS.md) | Toast notification system, PDF download feedback, user notifications | Frontend developers, UX designers |
+| 14 | [BACKEND-HANDLERS.md](14-BACKEND-HANDLERS.md) | Handler architecture, type safety, middleware pattern, testing strategy | Backend developers |
 
 ### User & Operations Documentation
 
@@ -141,4 +143,4 @@ All documentation in this project follows these standards:
 
 **Last Updated**: 2025-11-20
 **Documentation Status**: ✅ Clean, organized, zero redundancy
-**Total Active Docs**: 13 core documents + archive
+**Total Active Docs**: 14 core documents + archive