refactor: consolidate lang into constants, rename services to email

- Merge lang package into constants (add IsValidLang, ValidateLang, AllLangs)
- Rename internal/services to internal/email for consistency with pdf package
- Rename types to avoid redundancy: EmailService→Service, EmailConfig→Config
- Update all imports and references across codebase
- Delete internal/lang directory (functions moved to constants)

internal/models/cv/loader.go

@@ -5,18 +5,18 @@ import (
 	"strings"
 	"time"
 
+	c "github.com/juanatsap/cv-site/internal/constants"
 	"github.com/juanatsap/cv-site/internal/fileutil"
-	"github.com/juanatsap/cv-site/internal/lang"
 )
 
 // LoadCV loads CV data from a JSON file for the specified language
 func LoadCV(language string) (*CV, error) {
-	if err := lang.Validate(language); err != nil {
+	if err := c.ValidateLang(language); err != nil {
 		return nil, err
 	}
 
 	var cvData CV
-	filename := fmt.Sprintf("data/cv-%s.json", language)
+	filename := fmt.Sprintf("%s/cv-%s.json", c.DirData, language)
 	if err := fileutil.LoadJSON(filename, &cvData); err != nil {
 		return nil, err
 	}