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/app/api/cron/agents/route.ts

@@ -0,0 +1,102 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { prisma } from '@/lib/prisma'
+import { calculateNextRun } from '@/lib/agents/schedule'
+
+export const dynamic = 'force-dynamic'
+
+/**
+ * POST /api/cron/agents
+ *
+ * Finds all enabled, non-manual agents whose nextRun <= now,
+ * executes them, and schedules the next run.
+ *
+ * Optional auth: set CRON_SECRET env var, callers must pass
+ * Authorization: Bearer <CRON_SECRET>
+ */
+export async function POST(request: NextRequest) {
+  // Optional auth
+  const cronSecret = process.env.CRON_SECRET
+  if (cronSecret) {
+    const authHeader = request.headers.get('authorization')
+    if (authHeader !== `Bearer ${cronSecret}`) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+    }
+  }
+
+  try {
+    const now = new Date()
+
+    const dueAgents = await prisma.agent.findMany({
+      where: {
+        isEnabled: true,
+        frequency: { not: 'manual' },
+        nextRun: { lte: now },
+      },
+      select: {
+        id: true,
+        userId: true,
+        frequency: true,
+        scheduledTime: true,
+        scheduledDay: true,
+        timezone: true,
+      },
+    })
+
+    if (dueAgents.length === 0) {
+      return NextResponse.json({ success: true, executed: 0 })
+    }
+
+    const results: { id: string; success: boolean; error?: string }[] = []
+
+    // Execute agents sequentially to avoid overwhelming the AI provider
+    for (const agent of dueAgents) {
+      try {
+        const { executeAgent } = await import('@/lib/ai/services/agent-executor.service')
+        const result = await executeAgent(agent.id, agent.userId)
+
+        // Calculate and set next run
+        const nextRun = calculateNextRun({
+          frequency: agent.frequency,
+          scheduledTime: agent.scheduledTime,
+          scheduledDay: agent.scheduledDay,
+          timezone: agent.timezone,
+        })
+
+        await prisma.agent.update({
+          where: { id: agent.id },
+          data: { nextRun },
+        })
+
+        results.push({ id: agent.id, success: result.success, error: result.error })
+      } catch (error) {
+        const msg = error instanceof Error ? error.message : 'Unknown error'
+        console.error(`[CronAgents] Agent ${agent.id} failed:`, msg)
+        results.push({ id: agent.id, success: false, error: msg })
+
+        // Still schedule next run even on failure
+        const nextRun = calculateNextRun({
+          frequency: agent.frequency,
+          scheduledTime: agent.scheduledTime,
+          scheduledDay: agent.scheduledDay,
+          timezone: agent.timezone,
+        })
+        await prisma.agent.update({
+          where: { id: agent.id },
+          data: { nextRun },
+        })
+      }
+    }
+
+    return NextResponse.json({
+      success: true,
+      executed: results.length,
+      results,
+    })
+  } catch (error) {
+    console.error('[CronAgents] Error:', error)
+    return NextResponse.json(
+      { success: false, error: 'Internal Server Error' },
+      { status: 500 }
+    )
+  }
+}