MUL-6625: enforce prepaid member seat capacity (#7491)

* MUL-6625: enforce prepaid member seat capacity

Co-authored-by: multica-agent <github@multica.ai>

* fix(seat-capacity): address review findings

Co-authored-by: multica-agent <github@multica.ai>

* fix(seat-capacity): close retry and worker races

Co-authored-by: multica-agent <github@multica.ai>

* fix(seat-capacity): bound lock waits during rollback

Co-authored-by: multica-agent <github@multica.ai>

* fix(seat-capacity): pin the cross-repo confirm settlement budget

Multica Cloud must wait longer than this worker's whole retry budget before it
treats a member row as one its ledger never recorded; adopting a member whose
confirm is still retrying counts one person as two seats, and an over-estimate
is what asks a customer to buy seats they do not owe.

That budget was only implicit here — 10 attempts of exponential backoff plus a
tick and a lock wait each — and Cloud's horizon had been set from a single
retry. Extract the backoff into retryBackoff, expose SettlementBudget, and
assert the worst case (29m15s) stays inside Cloud's 45m horizon, so raising
maxAttempts or the backoff cap fails review here instead of widening the window
in which Cloud cannot quantify a ledger.

Co-authored-by: multica-agent <github@multica.ai>

* Revert "fix(seat-capacity): pin the cross-repo confirm settlement budget"

This reverts commit d0ec36f647139ee30244b0ac9b705cf91a75e46d.

Co-authored-by: multica-agent <github@multica.ai>

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Co-authored-by: Sol-Boy <sol-boy@multica-ai.local>
Co-authored-by: multica-agent <github@multica.ai>