feat: implement agent scheduled execution with cron and time picker

- Add scheduledTime, scheduledDay, timezone fields to Agent schema
- Create calculateNextRun() helper with timezone-aware scheduling
- Add POST /api/cron/agents endpoint for external scheduler
- Calculate nextRun on agent create, update, and after execution
- Add time/day picker in agent form (daily/weekly/monthly)
- Show "Next run" countdown in agent card
- Add i18n keys for schedule UI (FR + EN)

External scheduler (N8N, Vercel Cron) should call /api/cron/agents
every 5-15 min. Requires `prisma db push` to apply schema changes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/agents/agent-card.tsx

@@ -38,6 +38,7 @@ interface AgentCardProps {
     isEnabled: boolean
     frequency: string
     lastRun: string | Date | null
+    nextRun?: string | Date | null
     createdAt: string | Date
     updatedAt: string | Date
     _count: { actions: number }
@@ -196,6 +197,16 @@ export function AgentCard({ agent, onEdit, onRefresh, onToggle }: AgentCardProps
           <span>{t('agents.metadata.executions', { count: agent._count.actions })}</span>
         </div>
 
+        {agent.frequency !== 'manual' && agent.nextRun && (
+          <div className="flex items-center gap-1.5 text-xs text-muted-foreground mb-3">
+            <Clock className="w-3 h-3" />
+            {t('agents.schedule.nextRun')}{' '}
+            {mounted
+              ? formatDistanceToNow(new Date(agent.nextRun), { addSuffix: true, locale: dateLocale })
+              : new Date(agent.nextRun).toISOString().split('T')[0]}
+          </div>
+        )}
+
         {lastAction && (
           <div className={`
             flex items-center gap-1.5 text-xs px-2 py-1 rounded-md mb-3