FL-1364 feat: per-recipient ExpiresAt in RewardAccrued

A multi-recipient accrual creates one lot per recipient, and the expiration period follows each recipient's loyalty tier — so two members in one sharing payout can hold different dates. The event-level ExpiresAt cannot describe that: the producer had to collapse it to the first recipient's date, and the display consumer then painted every row of the payout with it.

RecipientReward gains its own ExpiresAt (additive, omitempty). The event-level field stays for single-recipient accruals and is documented as such.

Nothing has shipped with the collapsed date: the pre-release consumers (WMS producer, LMS statement) are updated in the same round.

Closes FL-1364