fix(stellar): per-payment expiry failure isolation, event-cursor correctness, callback idempotency

[tegaReed-crypto]

expireOldPayments aborts the whole batch on one failure

• StellarMonitorService.expireOldPayments looped over expired payments calling
  stellar.invokeContract('expire', ...) with no per-iteration try/catch, so a
  single transient Soroban RPC failure aborted the entire batch and silently
  delayed expiry of every subsequent payment until the next ~30s cycle.
• Each iteration is now wrapped in its own try/catch. On failure we log the error
  and raise a deduped STELLAR_MONITOR admin alert, then continue processing the
  remaining payments in the batch (mirroring the existing confirmPayment error path).

SorobanEventIndexer advances cursor past failed events

• SorobanEventIndexer.pollEvents set latestSeenLedger from the RPC response's
  latestLedger before processing events and persisted that cursor regardless of
  whether some events failed to parse/dispatch and were sent to the DLQ.
• The persisted cursor now advances only to the ledger of the last successfully
  dispatched event (or the RPC latestLedger for an empty page, where there is
  nothing to reprocess), so failed/DLQ'd events are re-fetched on subsequent polls
  instead of being silently skipped.

handlePartnerCallback has no idempotency check

• SettlementsService.handlePartnerCallback looked up the settlement by
  payload.reference and unconditionally processed it, so retried partner webhooks
  re-invoked stellar.invokeContract('settle', ...), re-dispatched
  payment.settled webhooks, and re-sent settlement-completed emails.
• The handler now short-circuits (logs and returns) when the settlement is already
  in the terminal state matching the callback intent (success +
  completed, or failed + failed).

Testing

• tsc --noEmit introduces no new errors in the changed files (remaining
  diagnostics are pre-existing in untouched spec/admin files).
• No new tests added, per instructions.

closes #192, closes #193, closes #194, closes #195.

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