chore: use common header parsing

internal/handler/helpers.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"log/slog"
 	"net/http"
-	"strings"
 )
 
 func writeJSON(w http.ResponseWriter, status int, v any) {
@@ -26,19 +25,3 @@ func decodeJSON(r *http.Request, v any) error {
 	defer r.Body.Close()
 	return json.NewDecoder(r.Body).Decode(v)
 }
-
-func csvHeader(r *http.Request, key string) []string {
-	raw := r.Header.Get(key)
-	if raw == "" {
-		return nil
-	}
-	parts := strings.Split(raw, ",")
-	out := make([]string, 0, len(parts))
-	for _, p := range parts {
-		p = strings.TrimSpace(p)
-		if p != "" {
-			out = append(out, p)
-		}
-	}
-	return out
-}