feat: Complete all remaining Future Improvements (#4-8)

Implemented 5 additional architectural improvements:

1. Response Types (types.go)
   - APIResponse with Success, Data, Error, Meta fields
   - ErrorInfo with Code, Message, Field, Details
   - MetaInfo with Timestamp, Version, RequestID
   - SuccessResponse() and NewErrorResponse() helpers
   - HealthCheckResponse for health endpoint
   - Consistent JSON API responses

2. Validation Tags (types.go)
   - Added struct tags to LanguageRequest
   - Added struct tags to PDFExportRequest
   - Declarative validation rules (oneof, required)
   - Self-documenting validation constraints
   - Ready for go-playground/validator integration

3. Context Helper Functions (middleware/preferences.go)
   - GetLanguage(), GetCVLength(), GetCVIcons(), GetCVTheme(), GetColorTheme()
   - IsLongCV(), IsShortCV() boolean helpers
   - ShowIcons(), HideIcons() boolean helpers
   - IsCleanTheme(), IsDefaultTheme() boolean helpers
   - IsDarkMode(), IsLightMode() boolean helpers
   - 13 new convenience functions for cleaner code

4. Typed Errors (errors.go)
   - ErrorCode constants for all error types
   - DomainError with Code, Message, Err, StatusCode, Field
   - Unwrap() support for error chains
   - WithError() and WithField() fluent builders
   - InvalidLanguageError(), InvalidLengthError(), etc.
   - PDFGenerationError(), MethodNotAllowedError(), RateLimitError()
   - 13 error codes, domain-specific constructors

5. Benchmark Tests
   - handlers/benchmarks_test.go (11 benchmarks)
   - middleware/benchmarks_test.go (12 benchmarks)
   - Sequential benchmarks for handlers, middleware, request parsing
   - Parallel benchmarks for concurrent load testing
   - Response creation benchmarks
   - Helper function benchmarks

Benefits:
- Type Safety: Validation tags and structured types
- Developer Experience: 13 context helpers reduce boilerplate
- Error Handling: Domain-specific errors with codes
- Performance Monitoring: 23 benchmarks for regression detection
- API Consistency: Standardized response formats
- Maintainability: Self-documenting validation and errors

Testing:
- All unit tests pass
- All benchmarks working
- Build succeeds
- No breaking changes

internal/handlers/errors.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"encoding/json"
+	"fmt"
 	"log"
 	"net/http"
 )
@@ -141,3 +142,136 @@ func DataLoadError(err error, dataType string) *AppError {
 		true,
 	)
 }
+
+// ==============================================================================
+// TYPED ERRORS
+// Domain-specific error types for better error handling
+// ==============================================================================
+
+// ErrorCode represents a specific error condition
+type ErrorCode string
+
+const (
+	ErrCodeInvalidLanguage   ErrorCode = "INVALID_LANGUAGE"
+	ErrCodeInvalidLength     ErrorCode = "INVALID_LENGTH"
+	ErrCodeInvalidIcons      ErrorCode = "INVALID_ICONS"
+	ErrCodeInvalidTheme      ErrorCode = "INVALID_THEME"
+	ErrCodeInvalidVersion    ErrorCode = "INVALID_VERSION"
+	ErrCodeTemplateNotFound  ErrorCode = "TEMPLATE_NOT_FOUND"
+	ErrCodeTemplateRender    ErrorCode = "TEMPLATE_RENDER"
+	ErrCodeDataLoad          ErrorCode = "DATA_LOAD"
+	ErrCodePDFGeneration     ErrorCode = "PDF_GENERATION"
+	ErrCodeMethodNotAllowed  ErrorCode = "METHOD_NOT_ALLOWED"
+	ErrCodeUnauthorized      ErrorCode = "UNAUTHORIZED"
+	ErrCodeForbidden         ErrorCode = "FORBIDDEN"
+	ErrCodeRateLimitExceeded ErrorCode = "RATE_LIMIT_EXCEEDED"
+)
+
+// DomainError represents a domain-specific error
+type DomainError struct {
+	Code       ErrorCode
+	Message    string
+	Err        error
+	StatusCode int
+	Field      string // Optional field that caused the error
+}
+
+// Error implements the error interface
+func (e *DomainError) Error() string {
+	if e.Err != nil {
+		return fmt.Sprintf("%s: %v", e.Code, e.Err)
+	}
+	return fmt.Sprintf("%s: %s", e.Code, e.Message)
+}
+
+// Unwrap returns the underlying error
+func (e *DomainError) Unwrap() error {
+	return e.Err
+}
+
+// NewDomainError creates a new domain error
+func NewDomainError(code ErrorCode, message string, statusCode int) *DomainError {
+	return &DomainError{
+		Code:       code,
+		Message:    message,
+		StatusCode: statusCode,
+	}
+}
+
+// WithError adds an underlying error
+func (e *DomainError) WithError(err error) *DomainError {
+	e.Err = err
+	return e
+}
+
+// WithField adds field information
+func (e *DomainError) WithField(field string) *DomainError {
+	e.Field = field
+	return e
+}
+
+// Common domain error constructors
+
+func InvalidLanguageError(lang string) *DomainError {
+	return NewDomainError(
+		ErrCodeInvalidLanguage,
+		fmt.Sprintf("Unsupported language: %s (use 'en' or 'es')", lang),
+		http.StatusBadRequest,
+	).WithField("lang")
+}
+
+func InvalidLengthError(length string) *DomainError {
+	return NewDomainError(
+		ErrCodeInvalidLength,
+		fmt.Sprintf("Unsupported length: %s (use 'short' or 'long')", length),
+		http.StatusBadRequest,
+	).WithField("length")
+}
+
+func InvalidIconsError(icons string) *DomainError {
+	return NewDomainError(
+		ErrCodeInvalidIcons,
+		fmt.Sprintf("Unsupported icons option: %s (use 'show' or 'hide')", icons),
+		http.StatusBadRequest,
+	).WithField("icons")
+}
+
+func InvalidThemeError(theme string) *DomainError {
+	return NewDomainError(
+		ErrCodeInvalidTheme,
+		fmt.Sprintf("Unsupported theme: %s (use 'default' or 'clean')", theme),
+		http.StatusBadRequest,
+	).WithField("theme")
+}
+
+func InvalidVersionError(version string) *DomainError {
+	return NewDomainError(
+		ErrCodeInvalidVersion,
+		fmt.Sprintf("Unsupported version: %s (use 'with_skills' or 'clean')", version),
+		http.StatusBadRequest,
+	).WithField("version")
+}
+
+func PDFGenerationError(err error) *DomainError {
+	return NewDomainError(
+		ErrCodePDFGeneration,
+		"Failed to generate PDF",
+		http.StatusInternalServerError,
+	).WithError(err)
+}
+
+func MethodNotAllowedError(method string) *DomainError {
+	return NewDomainError(
+		ErrCodeMethodNotAllowed,
+		fmt.Sprintf("Method %s not allowed", method),
+		http.StatusMethodNotAllowed,
+	)
+}
+
+func RateLimitError() *DomainError {
+	return NewDomainError(
+		ErrCodeRateLimitExceeded,
+		"Rate limit exceeded. Please try again later.",
+		http.StatusTooManyRequests,
+	)
+}