DateTime handling rules

Overview

This document defines rules for handling date/time across all layers. UTC is the single source of truth for all internal operations.

Key Principle: Store and process in UTC. Convert to KST only at the final display boundary. All controller inputs must be UTC.

Rule	Description
Internal timezone	UTC everywhere (JVM, DB, domain logic)
Controller input	Must be UTC. If KST arrives, convert to UTC immediately
Controller output	UTC by default. Convert to KST only when display requires it
KST conversion	Use `.toKst()` extension only at the response boundary
Utilities	Use `io.glory.common.utils.datetime` for all datetime operations