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/lib/ai/services/agent-executor.service.ts

@@ -14,6 +14,7 @@ import { toolRegistry } from '../tools'
 import { sendEmail } from '@/lib/mail'
 import { getAgentEmailTemplate } from '@/lib/agent-email-template'
 import { extractAndDownloadImages, extractImageUrlsFromHtml, downloadImage } from '../tools/extract-images'
+import { calculateNextRun } from '@/lib/agents/schedule'
 
 // Import tools for side-effect registration
 import '../tools'
@@ -1094,9 +1095,19 @@ export async function executeAgent(agentId: string, userId: string, promptOverri
       }
     }
 
+    const nextRunUpdate: Record<string, Date | null> = {}
+    if (agent.frequency !== 'manual') {
+      nextRunUpdate.nextRun = calculateNextRun({
+        frequency: agent.frequency,
+        scheduledTime: agent.scheduledTime,
+        scheduledDay: agent.scheduledDay,
+        timezone: agent.timezone,
+      })
+    }
+
     await prisma.agent.update({
       where: { id: agentId },
-      data: { lastRun: new Date() }
+      data: { lastRun: new Date(), ...nextRunUpdate }
     })
 
     if (result.success && agent.notifyEmail) {