Apply the post-tax-filing lock to an assigned customer's closed GL period (irreversible)

Marks a closed period immutable after its tax return was filed elsewhere. It does not submit or verify a return. Irreversible: there is no unlock. Writes an audit_logs entry (action file-lock) with the actor and time.

Requires accountant access to the customer AND the company admin/owner role that /companies/{companyId}/gl-periods/{id}/file-lock requires; assignment alone is not enough (#2104). Signed-in humans only: API keys get 403 and the service principal gets 404.