auth: default admin/admin credentials on first install + bump 1.0.4

SeedAdmin now uses 'admin'/'admin' as defaults when environment
variables are not set, so the service works out-of-the-box after
first install.

internal/auth/seed.go

@@ -6,8 +6,11 @@ import (
 )
 
 func SeedAdmin(db *sql.DB, username, password string) error {
-	if username == "" || password == "" {
-		return nil
+	if username == "" {
+		username = "admin"
+	}
+	if password == "" {
+		password = "admin"
 	}
 
 	var exists bool