docs: Add comprehensive documentation for architectural enhancements

Created detailed documentation for all 5 architectural improvements:

Educational Documentation (_go-learning/):
- Created 005-architectural-enhancements.md (900+ lines)
- Detailed explanation of each enhancement
- Code examples and usage patterns
- Before/after comparisons
- Benefits and interview talking points
- Future considerations

Public Documentation (doc/):
- Updated 14-BACKEND-HANDLERS.md
- Added "Architectural Enhancements" section
- Response Types with examples
- Validation Tags guide
- Context Helpers usage
- Typed Errors documentation
- Performance Benchmarks guide
- Updated table of contents
- Updated changelog

Documentation Coverage:
- Response Types: Structure, helpers, usage examples
- Validation Tags: Declarative rules, self-documenting
- Context Helpers: 13 functions documented
- Typed Errors: 13 error codes, constructors, usage
- Benchmarks: 23 benchmarks, running instructions

All improvements now fully documented for:
- Internal learning and interviews
- Public consumption and contribution
- Developer onboarding
- Architecture understanding

doc/14-BACKEND-HANDLERS.md

@@ -14,6 +14,12 @@ This document explains how the backend handles HTTP requests, focusing on the ha
 4. [Testing Strategy](#testing-strategy)
 5. [Data Flow](#data-flow)
 6. [Best Practices](#best-practices)
+7. [Architectural Enhancements](#architectural-enhancements)
+   - [Response Types](#response-types)
+   - [Validation Tags](#validation-tags)
+   - [Context Helpers](#context-helpers)
+   - [Typed Errors](#typed-errors)
+   - [Performance Benchmarks](#performance-benchmarks)
 
 ---
 
@@ -551,6 +557,165 @@ func TestInvalidLanguage(t *testing.T) {
 
 ---
 
+## Architectural Enhancements
+
+### Response Types
+
+The handler layer uses standardized response types for consistent API responses:
+
+```go
+// APIResponse - Standardized wrapper for JSON responses
+type APIResponse struct {
+    Success bool        `json:"success"`
+    Data    interface{} `json:"data,omitempty"`
+    Error   *ErrorInfo  `json:"error,omitempty"`
+    Meta    *MetaInfo   `json:"meta,omitempty"`
+}
+
+// ErrorInfo - Structured error information
+type ErrorInfo struct {
+    Code    string `json:"code"`              // Error code
+    Message string `json:"message"`           // Human-readable message
+    Field   string `json:"field,omitempty"`   // Field that caused error
+    Details string `json:"details,omitempty"` // Additional details
+}
+```
+
+**Helper Functions**:
+- `SuccessResponse(data)` - Create success response
+- `NewErrorResponse(code, message)` - Create error response
+- `ErrorResponseWithField(code, message, field)` - Error with field info
+
+### Validation Tags
+
+Request types use struct tags for declarative validation:
+
+```go
+type PDFExportRequest struct {
+    Lang    string `validate:"required,oneof=en es"`
+    Length  string `validate:"required,oneof=short long"`
+    Icons   string `validate:"required,oneof=show hide"`
+    Version string `validate:"required,oneof=with_skills clean"`
+}
+```
+
+**Benefits**:
+- Self-documenting validation rules
+- Ready for validator library integration
+- Centralized validation logic
+- Easy to extend
+
+### Context Helpers
+
+The middleware provides 13 convenience functions for cleaner code:
+
+```go
+// Getters
+middleware.GetLanguage(r)      // Get language preference
+middleware.GetCVLength(r)      // Get CV length preference
+middleware.GetCVTheme(r)       // Get theme preference
+
+// Boolean helpers
+middleware.IsLongCV(r)         // True if long CV format
+middleware.ShowIcons(r)        // True if icons visible
+middleware.IsCleanTheme(r)     // True if clean theme
+middleware.IsDarkMode(r)       // True if dark mode
+```
+
+**Usage**:
+```go
+// Before
+prefs := middleware.GetPreferences(r)
+if prefs.CVLength == "long" {
+    // ...
+}
+
+// After
+if middleware.IsLongCV(r) {
+    // ...
+}
+```
+
+### Typed Errors
+
+Domain-specific errors with error codes for programmatic handling:
+
+```go
+// Error codes
+type ErrorCode string
+
+const (
+    ErrCodeInvalidLanguage   ErrorCode = "INVALID_LANGUAGE"
+    ErrCodeInvalidLength     ErrorCode = "INVALID_LENGTH"
+    ErrCodePDFGeneration     ErrorCode = "PDF_GENERATION"
+    ErrCodeRateLimitExceeded ErrorCode = "RATE_LIMIT_EXCEEDED"
+    // ... 13 total error codes
+)
+
+// DomainError with context
+type DomainError struct {
+    Code       ErrorCode
+    Message    string
+    Err        error
+    StatusCode int
+    Field      string
+}
+```
+
+**Constructors**:
+```go
+InvalidLanguageError(lang)    // Returns typed error with code
+PDFGenerationError(err)       // Wraps underlying error
+RateLimitError()              // Rate limit exceeded
+```
+
+**Usage**:
+```go
+// Create typed error
+return InvalidLanguageError("fr")
+// Returns: INVALID_LANGUAGE: Unsupported language: fr (use 'en' or 'es')
+
+// Error chaining
+return PDFGenerationError(err).WithError(originalErr)
+```
+
+### Performance Benchmarks
+
+Comprehensive benchmark suite for performance monitoring:
+
+**Handlers** (11 benchmarks):
+- `BenchmarkHome` - Home page handler
+- `BenchmarkCVContent` - Content rendering
+- `BenchmarkToggleLength` - Toggle handlers
+- `BenchmarkParsePDFExportRequest` - Request parsing
+- `BenchmarkPrepareTemplateData` - Data preparation
+- `BenchmarkParallelHome` - Parallel load test
+- Response creation benchmarks
+
+**Middleware** (12 benchmarks):
+- `BenchmarkPreferencesMiddleware` - Middleware performance
+- `BenchmarkGetPreferences` - Context retrieval
+- `BenchmarkGetLanguage` - Helper functions
+- `BenchmarkIsLongCV` - Boolean helpers
+- `BenchmarkParallelPreferencesMiddleware` - Concurrent load
+
+**Running Benchmarks**:
+```bash
+# All benchmarks
+go test -bench=. ./internal/handlers/... ./internal/middleware/...
+
+# Specific benchmark with memory stats
+go test -bench=BenchmarkHome -benchmem ./internal/handlers/...
+
+# Compare for regression detection
+go test -bench=. -benchmem ./... > baseline.txt
+# Make changes
+go test -bench=. -benchmem ./... > current.txt
+benchcmp baseline.txt current.txt
+```
+
+---
+
 ## Related Documentation
 
 - [Architecture Overview](./1-ARCHITECTURE.md) - System architecture patterns
@@ -563,4 +728,5 @@ func TestInvalidLanguage(t *testing.T) {
 
 ## Changelog
 
+- **Nov 20, 2024**: Added architectural enhancements section (response types, validation tags, context helpers, typed errors, benchmarks)
 - **Nov 20, 2024**: Initial documentation covering handler refactoring, type safety, middleware pattern, and testing strategy