Replace availability rules and/or date overrides for a schedule

Replace weekly availability rules and/or date overrides for a schedule.

Body: {"availability": [...], "overrides": [...]}

• availability: list of {days, start_time, end_time} - replaces ALL weekly rules.
• overrides: list of {date, start_time, end_time} - replaces ALL date overrides.

Omit a key to leave that set unchanged. At least one key is required. Mirrors the in-app PATCH /schedules/{id}/availability logic verbatim. Typed via UpdateAvailabilityRequest so malformed items fail at FastAPI validation (422) rather than as an opaque 500.