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/locales/en.json

@@ -1254,6 +1254,21 @@
       "weekly": "Weekly",
       "monthly": "Monthly"
     },
+    "schedule": {
+      "nextRun": "Next run",
+      "time": "Time",
+      "dayOfWeek": "Day of week",
+      "dayOfMonth": "Day of month",
+      "days": {
+        "mon": "Monday",
+        "tue": "Tuesday",
+        "wed": "Wednesday",
+        "thu": "Thursday",
+        "fri": "Friday",
+        "sat": "Saturday",
+        "sun": "Sunday"
+      }
+    },
     "status": {
       "success": "Succeeded",
       "failure": "Failed",