docs(api): calendar event days, range delete and optional summary

[ysyneu]

What

Bring the calendar event API reference and the service calendar guide in line with current behavior.

API reference (openapi.{en,zh}.json, on-call.openapi.{en,zh}.json)

• POST /calendar/event/list: response adds days, keyed by YYYY-MM-DD, each a new CalDay (is_off, event_id, layer = custom / holiday / weekly). The description states the resolution order: the calendar's own events, then linked holiday calendars, then weekly workdays; inside a layer a working-day event beats a day off.
• POST /calendar/event/delete: event_id is now optional; start_at + end_at clear the calendar's own events off [start_at, end_at). Exactly one of the two forms.
• POST /calendar/event/upsert: summary is optional (no minLength); the description covers how a write trims the calendar's other own events on the same days, and that end_at must be later than start_at.

Guide ({zh,en}/on-call/configuration/service-calendar.mdx)

• "Adjust dates" row describes selecting a day or a range and the action bar buttons (labels match the console).
• New FAQ: which setting wins when a day has several, what "Reset to default" does, and what the colors mean.

Checks

• python3 scripts/lint_openapi.py → OK: 12 spec files, no violations
• mint broken-links → no broken links