feat(payroll): add payroll v1 skeleton (track C)

[rmak78]

Summary

Implements PR #12 – Track C by adding a payroll foundation across schema + API.

What’s included

• Added D1 migration: data-model/migrations/0007_payroll_v1_skeleton.sql
  • pay_cycles
  • pay_components
  • payroll_runs
  • supporting indexes
• Added API endpoints in src/index.ts
  • GET/POST /v1/pay-cycles
  • GET/POST /v1/pay-components
  • GET/POST /v1/payroll-runs
• Registered payroll endpoints in:
  • /v1 route listing
  • write-protected path set (API key checks on POST)
  • modules metadata (hris now includes payroll entities)
• Updated README with new endpoints and delivered progress note.

Validation

• Ran deployment validation: npx wrangler deploy --dry-run ✅

Notes

• This is intentionally a v1 skeleton: schema + create/list APIs only, ready for later calculation and posting logic.
• No merge performed as requested.