feat: add tag-based validation system with reflection caching

Implement a declarative struct tag validation system for Go:

- Add validator.go with sync.Map caching for reflection metadata
- Add rules.go with 11 built-in validation rules (required, email,
  pattern, honeypot, timing, etc.)
- Add errors.go with FieldError and ValidationErrors types
- Update ContactFormRequest with validate tags
- Add ValidateContactFormV2() using the new tag-based validator

Rules implemented:
- required/optional: field presence validation
- trim/sanitize: automatic value transformations
- min/max: UTF-8 aware length validation
- email: RFC 5322 email format validation
- pattern: predefined regex patterns (name, subject, company)
- no_injection: email header injection prevention
- honeypot: bot trap (must be empty)
- timing: timestamp validation for bot detection

Documentation:
- docs/go-validation-system.md: complete validation guide
- docs/go-template-system.md: template manager documentation
- docs/go-routes-api.md: routes and API reference
- docs/README.md: documentation index

internal/validation/errors.go

@@ -0,0 +1,65 @@
+package validation
+
+import "strings"
+
+// FieldError represents a single field validation error
+type FieldError struct {
+	Field   string `json:"field"`            // Field name
+	Tag     string `json:"tag"`              // Validation tag that failed
+	Param   string `json:"param,omitempty"`  // Optional parameter (e.g., "100" for "max=100")
+	Message string `json:"message"`          // Human-readable error message
+}
+
+// Error implements the error interface
+func (e FieldError) Error() string {
+	if e.Param != "" {
+		return e.Field + ": " + e.Message + " (" + e.Tag + "=" + e.Param + ")"
+	}
+	return e.Field + ": " + e.Message
+}
+
+// ValidationErrors represents multiple field validation errors
+type ValidationErrors []FieldError
+
+// Error implements the error interface
+func (ve ValidationErrors) Error() string {
+	if len(ve) == 0 {
+		return ""
+	}
+
+	var sb strings.Builder
+	sb.WriteString("validation failed: ")
+	for i, err := range ve {
+		if i > 0 {
+			sb.WriteString(", ")
+		}
+		sb.WriteString(err.Error())
+	}
+	return sb.String()
+}
+
+// HasErrors returns true if there are any validation errors
+func (ve ValidationErrors) HasErrors() bool {
+	return len(ve) > 0
+}
+
+// GetFieldError returns the first error for a specific field, or nil if none
+func (ve ValidationErrors) GetFieldError(field string) *FieldError {
+	for _, err := range ve {
+		if err.Field == field {
+			return &err
+		}
+	}
+	return nil
+}
+
+// GetFieldErrors returns all errors for a specific field
+func (ve ValidationErrors) GetFieldErrors(field string) []FieldError {
+	var errors []FieldError
+	for _, err := range ve {
+		if err.Field == field {
+			errors = append(errors, err)
+		}
+	}
+	return errors
+}