Fix scheduler idle writes when monitors are paused

[VrianCao]

Summary

• Add a schedulable-monitor preflight so paused/inactive monitors skip the monitor check pipeline.
• Avoid scheduler lease/check-result/state/outage writes when no active non-paused monitors exist.
• Keep idle maintenance notifications and public snapshot refresh work running so homepage/status/artifact snapshots do not expire.
• Add regression coverage for all-paused idle ticks and post-lease pause races.

Root Cause

When all monitors were paused, the scheduler could still enter the no-due-monitor path and queue public snapshot/fragment work after acquiring the scheduler lease. That avoided monitor checks but still produced D1 writes unrelated to active monitoring.

Verification

• pnpm --filter @uptimer/worker exec vitest run --config vitest.config.ts test/scheduled.test.ts
• pnpm --filter @uptimer/worker typecheck
• pnpm lint
• pnpm typecheck
• pnpm test
• git diff --check

Fixes #87