tools/createuser: report which field collided on insert failure

ON CONFLICT (id) DO NOTHING only catches the id itself, so every other
failure (duplicate username/phone/signup_email) was reported as a generic
"already exists (or insert failed)" - not useful for telling apart the four
distinct causes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/createuser/main.go

@@ -27,11 +27,14 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/binary"
+	"errors"
 	"flag"
 	"fmt"
 	"os"
 	"time"
 
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"telesrv/internal/config"
@@ -124,7 +127,7 @@ func main() {
 
 	var createdID int64
 	if err := row.Scan(&createdID); err != nil {
-		fmt.Fprintf(os.Stderr, "createuser: id %d already exists (or insert failed): %v\n", *id, err)
+		fmt.Fprintln(os.Stderr, describeInsertFailure(*id, *username, displayPhone, signupEmail, err))
 		os.Exit(1)
 	}
 
@@ -132,6 +135,31 @@ func main() {
 		createdID, accessHash, *firstName, *lastName, *username, displayPhone, signupEmail)
 }
 
+// describeInsertFailure turns the INSERT's failure into a message naming the
+// actual thing that collided, instead of "id already exists" for every case:
+// ON CONFLICT (id) DO NOTHING only covers the id itself, so a duplicate
+// username/phone/signup_email surfaces here as a distinct unique-violation
+// error (pgx.ErrNoRows only means the id itself was the conflict).
+func describeInsertFailure(id int64, username, phone, signupEmail string, err error) string {
+	var pgErr *pgconn.PgError
+	if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+		switch pgErr.ConstraintName {
+		case "users_username_lower_unique_idx":
+			return fmt.Sprintf("createuser: username %q is already taken", username)
+		case "users_phone_unique_idx":
+			return fmt.Sprintf("createuser: phone %q is already in use", phone)
+		case "users_signup_email_lower_unique_idx":
+			return fmt.Sprintf("createuser: email %q is already in use by another account", signupEmail)
+		default:
+			return fmt.Sprintf("createuser: unique constraint %q violated: %v", pgErr.ConstraintName, err)
+		}
+	}
+	if errors.Is(err, pgx.ErrNoRows) {
+		return fmt.Sprintf("createuser: id %d already exists", id)
+	}
+	return fmt.Sprintf("createuser: insert failed: %v", err)
+}
+
 // assignEmailSignupDisplayPhone mirrors internal/app/auth/service.go's method
 // of the same name: pick a random "888"-prefixed display phone and re-roll on
 // the astronomically unlikely collision with an existing account's phone.