fix: fire a one-time schedule on the exact chosen date and time (#1497)

Simple "Once" schedules compiled to a 5-field cron, which has no year field, so
the scheduler computed the next run as the next annual occurrence. A date chosen
for a later year ran a year early, and a time already elapsed today ran a year
late, contradicting the UI promise that the task fires on the chosen date.

One-time schedules now send the chosen absolute timestamp (run_at) and the
backend pins next_run_at to it instead of the cron-derived next run, so the run
fires on the exact selected instant including the year. The Simple-mode
validation now compares the full chosen instant against the current time, so a
time earlier today is rejected as past rather than silently deferred a year.
run_at is validated as a finite, future epoch-millisecond timestamp on create
and update. The enable/disable toggle preserves a one-shot's pinned next_run_at
(its yearless cron cannot reconstruct the chosen year), so re-enabling restores
the exact instant. Recurring shapes and Advanced mode are unchanged (cron stays
authoritative).

frontend/src/components/ScheduledOperationsView.tsx

@@ -22,6 +22,7 @@ import {
   buildCron,
   parseCron,
   getSimpleScheduleError,
+  getOnceRunAt,
   type SimpleSchedule,
 } from '@/lib/scheduling';
 import { ScheduleSimplePanel } from './ScheduleSimplePanel';
@@ -277,6 +278,11 @@ export default function ScheduledOperationsView({ filterNodeId, onClearFilter, p
     // backend stores; Advanced mode sends the raw expression as-is.
     const cronExpression = scheduleMode === 'simple' ? buildCron(simpleSchedule) : formCron;
 
+    // A one-time ('once') Simple schedule pins its exact run instant (including
+    // year) via run_at, because the 5-field cron cannot encode a year. null for
+    // every recurring shape and for Advanced mode, where the cron is authoritative.
+    const runAt = scheduleMode === 'simple' ? getOnceRunAt(simpleSchedule) : null;
+
     const body: Record<string, unknown> = {
       name: formName,
       target_type: actionDef.targetType,
@@ -284,6 +290,7 @@ export default function ScheduledOperationsView({ filterNodeId, onClearFilter, p
       cron_expression: cronExpression,
       enabled: formEnabled,
       delete_after_run: formDeleteAfterRun,
+      run_at: runAt,
       target_id: actionDef.requiresStack ? formTargetId : null,
       node_id: actionDef.requiresNode && formNodeId ? parseInt(formNodeId, 10) : null,
       prune_targets: formAction === 'prune' && formPruneTargets.length > 0 ? formPruneTargets : null,