feat: add tick runner and region reset clock

[ggmolly]

Summary

• Adds a scheduler foundation with a reusable periodic tick runner for background systems.
• Adds a region-aware reset clock that computes daily, weekly, and monthly reset boundaries in UTC.
• Provides reset helpers for current region resolution and month key derivation for timed systems.

Changes

• Introduced internal/scheduler/runner.go with Runner, configurable tick interval, and per-tick job dispatch.
• Introduced internal/scheduler/reset_clock.go with region-based reset calculations for CN/EN/JP/KR/TW.
• Added focused unit coverage in internal/scheduler/runner_test.go and internal/scheduler/reset_clock_test.go.