fix(backend): seed admin tolerates only unique-violation, propagates other errors

backend/internal/seed/seed.go

@@ -2,8 +2,11 @@ package seed
 
 import (
 	"context"
+	"errors"
 	"log"
 
+	"github.com/jackc/pgx/v5/pgconn"
+
 	"booklib/internal/auth"
 	"booklib/internal/store"
 )
@@ -21,8 +24,12 @@ func Admin(ctx context.Context, s *store.Store, user, pass string) error {
 		return err
 	}
 	if _, err := s.CreateUser(ctx, user, h, "admin"); err != nil {
-		log.Printf("seed admin %q may already exist: %v", user, err)
-		return nil
+		var pgErr *pgconn.PgError
+		if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+			log.Printf("seed admin %q may already exist: %v", user, err)
+			return nil
+		}
+		return err
 	}
 	log.Printf("seeded initial admin user %q", user)
 	return nil