refactor(backend): migrate from sql.ErrNoRows to pgx.ErrNoRows
Replace all database/sql.ErrNoRows checks with pgx.ErrNoRows across backend handlers.
Migration includes: jwt.go, local.go, admin_reserve.go, custom_services.go,
discount_campaigns.go, services.go, account.go, customer_relationship.go,
guest.go. Also removes unused test_helpers.go resetTestData function.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>