Add structured log for failed alert threshold

Implemented a try/catch inside evaluatePriceAlertsForMovement (src/modules/alerts/alert.service.ts:132) that emits a structured logger.error with creator_id, ledger_sequence, new_price, error_message, and failed_at when the alert threshold check fails (e.g., database error reading alerts). Added optional ledger_sequence to the PriceMovement type so the log carries enough context for an operator to replay the check. The error is re-thrown after logging so existing error propagation is preserved. New tests verify the log is emitted with all fields on DB failure and is not emitted on success.
Closed #510

Author: Godfrey-Delight