Add timesheet sync: pull actual hours, daily cron, per-cell source indicators

- fetchTimesheetShifts: hits /api/v2/shifts, subtracts breaks, carries PENDING/APPROVED status
- syncJobs.js: shared runRotaSync/runTimesheetSync logic used by routes and cron
- Rota sync now skips dates already marked source='timesheet'
- POST /api/workforce/sync-timesheets: on-demand pull of last 7 days actual hours
- Daily 6am cron: rota sync then timesheet sync, each logged independently
- upsertWorkforceShifts now writes source per row (workforce or timesheet)
- Frontend: Pull timesheets button alongside Sync rota button
- Clock icon on timesheet cells — amber for PENDING, green for APPROVED
- Both buttons disabled while either sync is in progress

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package.json

@@ -11,6 +11,7 @@
     "@fastify/cors": "^9.0.1",
     "fastify": "^4.28.1",
     "jose": "^5.9.6",
+    "node-cron": "^4.6.0",
     "pg": "^8.13.1"
   }
 }