fix(worker): start uptime windows from first probe

[Lumexia]

Summary

Fixes incorrect UNKNOWN accounting for newly created monitors before their first probe.
The uptime window now starts at the first observed check within range, so pre-probe time is no longer treated as UNKNOWN/unavailable.

Changes

• Updated synthetic "today" uptime computation in apps/worker/src/public/status.ts to use an effective range start based on first probe time.
• Added last_checked_at context into uptime window resolution to distinguish "never checked" monitors.
• Unified public uptime endpoint behavior in apps/worker/src/routes/public.ts:
  • /api/v1/public/monitors/:id/uptime
  • /api/v1/public/analytics/uptime (partial-day path)
• Added/updated regression coverage in apps/worker/test/public-status.test.ts:
  • First-probe start behavior
  • Never-checked monitor behavior
  • Pre-creation downtime not counted

Test Plan

• pnpm lint passes
• pnpm typecheck passes
• pnpm test passes
• Manually tested