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/prisma/schema.prisma

@@ -275,6 +275,9 @@ model Agent {
   frequency        String        @default("manual") // manual, hourly, daily, weekly, monthly
   lastRun          DateTime?
   nextRun          DateTime?
+  scheduledTime    String?       @default("08:00") // HH:mm in user's local time
+  scheduledDay     Int?          // 0-6 for weekly (Mon=0), 1-31 for monthly
+  timezone         String?       // IANA timezone, e.g. "Europe/Paris"
   isEnabled        Boolean       @default(true)
   targetNotebookId String?
   sourceNotebookId String?       // For monitor type: notebook to watch