Replace a team's schedule

Covers in force over this schedule.

Carried on the schedule rather than fetched at each call site because every resolution has to see them. An override the page path forgets to load is worse than no override feature at all: the engineer who arranged cover stops watching, and the page still goes to them.

Stored in oncall_overrides, not in the schedule row — they have their own lifecycle and audit trail.

Absences in force over this schedule, for the people on its rotations.

Carried here for the same reason as the covers, against the same failure: a path that loads the schedule and forgets the absences gets a plausible answer that pages somebody on a beach. Stored org-wide in oncall_unavailability and narrowed to this schedule's members by the loader.