Scheduling and reminders module: time-based triggers for any module

Part of #249.

Time-based triggers any module can hang work off: send this later, remind me then, run this on a schedule. Distinct from the workflow engine (which reacts to events); this reacts to time.

Three unrelated systems that reuse it: a CRM (follow-up reminder), a CMS (scheduled publish, which already exists ad hoc), a survey (close at a deadline). Primitive.

Contract:
- A scheduled entry with a due time, a target action, and a status, plus recurrence.
- A single sweep (the scheduled-content service is the seed of this) that fires due entries and hands them to the raising module, tenant-scoped.
- At-least-once with idempotency, so a missed sweep or a retry does not double-fire.

Done when: a module ships with tests (schedule, fire at due time with a fake clock, recurrence, idempotent retry, tenant isolation), and the existing content scheduler is refactored to sit on it rather than beside it.