feat(alerts): on-call schedules in the dashboard: who is on call, a calendar, overrides, and paging people - #2307

🟢 Confidence 10/10 · safe to merge. Both earlier findings are fixed and the new behavior is pinned by scene tests; the schedule pages' model, message and command modules went unread.
_{no findings · tests covered · risk medium}

Adds on-call schedules to the alerts dashboard: On-call tab and inbox strip, a schedule form, a weekly calendar with overrides, people and schedule targets in escalation chains, and paged-people timeline entries. Both earlier findings are fixed; the McpQueryError issues in page/registry.ts are unrelated and untouched.

• oncallTab and whoIsOnCallStrip join the alerts hub, between SLOs and Routing
• New AlertsSchedule page: week calendar, overrides, DST-aware day grid
• New schedule form with layers, rotations, handoff and weekly windows
• Escalation steps now target destinations, people and schedules via pickList

Fixed since the last review

• ✅ F21 · Every layer's rotation control carries the same accessible name
• ✅ F22 · listOverrides reads one page, so overrides past the first 100 are unreachable

Production impact

_{b9733a8 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}