Model recurrence_schedule reads on schedule entries (both projections)

Split out of #374's schedule-entry audit; deliberately NOT part of the ReplaceScheduleEntry triad.

`recurrence_schedule` is emitted on schedule entries only when recurring, and the occurrence show emits it always — two projections of a nested structure (frequency, days, week_instance, start/end dates) that is unmodeled in the spec. `recurs_until` is readable only as `recurrence_schedule.end_date` (write key differs). Recurring entries are read-only for the SDK today (the flat update route 302s), so this is pure read modeling: add the nested structure, attach it optional-nullable to `ScheduleEntry` (and the occurrence projection if/when modeled), fixtures with both projections.

🤖 _Drafted with agent assistance; filed by @jeremy._

Locate the ScheduleEntry model, any occurrence projection model, and existing schedule-entry fixtures; start by comparing recurring schedule-entry and occurrence JSON responses. Done means the optional-nullable recurrence_schedule structure is represented for the applicable read projections, with fixtures covering both forms and the end_date mapping for recurs_until.

Written by the indexing model from the issue text.