Locks or unlocks one or more calendar days for a specific user on a given Time Clock. The isLocked boolean on the request body selects the operation: true (the default) prevents subsequent time-activity edits on those dates; false removes the per-day lock and allows edits again. Idempotent: dates already in the requested state are no-ops, and the response reflects the final state. This endpoint does not reopen approved payroll periods. If isLocked is false and any of the requested dates would actually transition from locked to unlocked but fall inside an approved period for this user, the entire request is rejected with DAYS_IN_APPROVED_PERIOD (409); no dates are unlocked. Approved periods must be reopened from the dashboard before calling this endpoint with isLocked: false. isLocked: true on an approved-period day is allowed and is a no-op for day-lock state.