Set Digest Prefs

Update the account's delivery shape. Returns the same payload as GET.

PATCH rather than a second PUT, and the split from PUT /agents/email is deliberate: that one changes the ADDRESS, which resets verification and sends mail. This one changes how much mail arrives at an already-verified address and must never do either of those things — a user toggling a checkbox has not asked to re-confirm their email.

A cadence or hour change re-anchors next_combined_digest_at to the next ALIGNED boundary from now (scheduler.aligned_digest_at), so somebody who switches to hourly at 12:40 gets their first hourly email at 13:00 rather than at 13:40 and every :40 thereafter.

combined_digest is accepted and ignored. The account digest is the only digest, so it is always on; the column is pinned TRUE here so a stale FALSE left by an older client can never make the scheduler skip the account.