Backdated Transaction into a Reopened Prior Period

negative) when a user without reopen authorization attempts to post into a closed period then 403 with error code period_closed.

Given a closed period, a controller with reopen authorization, and a correcting journal entry with an effective_date inside the closed period;
when the controller invokes the reopen workflow with a reason and posts the backdated entry;
then the entry posts at the backdated effective_date, downstream period balances re-derive, affected already-issued statements are flagged for restatement, the period is re-locked, and the reopen is audit-logged with actor + reason;
(negative) when a user without reopen authorization attempts to post into a closed period then 403 with error code period_closed.

accounting_period { id: string, entity_id: string, period_start: date, period_end: date, status: enum, reopened_by: string, reopen_reason: string, reopened_at: timestamp, locked_at: timestamp, external_id: string };
references journal_entry, restatement_flag;
(reference, product may differ).

POST /v1/accounting-periods/{id}/reopen { reason } -> 200 { id, status: OPEN };
POST /v1/journal-entries { effective_date, ... } -> 201 { id, effective_date };
POST /v1/accounting-periods/{id}/lock -> 200 { status: CLOSED };
emits period.reopened, period.locked events;
audit-logged with actor.

OPEN -> CLOSED -> OPEN (reopen) -> CLOSED;
guard: posting into CLOSED period blocked without active reopen workflow;
reopen requires role gate + reason;
re-lock required after backdated entry;
concurrency-safe against in-flight current-period postings.