fix: wire backend payroll and auth flows

[ChaoLing140]

This PR bundles the backend-side work for the four linked issues:

• JWT auth and org-scoped RBAC enforcement
• Organization-locked balance and preflight checks
• Payroll schedule persistence with timezone-aware next-run tracking
• Bulk payment batching with stop-on-failure rollback behavior

Closes #753
Closes #754
Closes #743
Closes #748

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@ChaoLing140 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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