Extract closing hours check into reusable checkClosingHours helper. Add repo.go for shared DB query helpers. Update admin_reserve to use closing_time and move overlap check inside transaction with FOR UPDATE.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
