Run the accrual detector for a period before locking it

The month-end close page's "Run detection" step (#1266). Runs the period-aware accrual detector for one GL period of this customer; the deferral detector is not run here (it runs on every sync). The lock does not depend on this call, and a successful lock starts the same sweep for its period in the background afterwards.

It never posts a journal entry, creates a schedule, approves a proposal, or changes close or period state. It can create proposals (some created blocked by the QA gate), supersede an older live proposal for the same candidate when its inputs changed or its real charge has posted, move an approved-but-unposted proposal to blocked when its real charge has posted, close the low-confidence review task of a proposal it supersedes, and write decision-manifest rows and audit events. If the detector fails midway, the proposals that run created are expired.

Bounded by a 20-second budget; a sweep that runs past it is reported as timed_out and may still finish afterwards. detection_off when accruals_deferrals_detection is off for the company (nothing runs). partial when some candidates could not be judged; skipped says why. A filing-locked period is refused with 422; open and closed periods are accepted.