feat: utility rebilling

[tusharshah21]

Summary

Implements variable utility billing reconciliation by allowing landlords to upload utility bill data, calculating each tenant’s share, and updating the tenant’s upcoming payment total for the next rent cycle.

Changes

• Added utility bill upload endpoint: POST /api/leases/:leaseId/utility-bills.
• Added tenant upcoming payment endpoint: GET /api/tenants/:tenantId/upcoming-payment.
• Added utility bill persistence and reconciliation logic in the DB layer:
  • stores uploaded utility bill entries
  • recalculates cycle-level utility share total
  • updates/inserts payment_schedules with rent + utility share
• Mounted payment routes under /api.
• Added focused tests in tests/utilityBillingReconciliation.test.js to verify:
  • share calculation
  • upcoming payment updates
  • cycle-gated tenant approval payload
  • landlord-lease authorization checks

Closes #14