feat(airdrops): enforce status lifecycle state machine and transitions (#87)

[ghzhost]

Summary

Closes #87

Implements a strict status state machine for airdrops across their lifecycle:

• Allowed transitions:
  • draft -> executing, cancelled, expired
  • executing -> completed, failed, cancelled, expired
  • Terminal statuses: completed, failed, cancelled, expired (cannot transition to any other status)
• Enforces transition validation via assertTransition(currentStatus, nextStatus) and transitionTo(id, nextStatus).
• Helper transition methods: markExecuting, markCompleted, markFailed, markExpired.
• Restricts expiry_ledger updates via update(id, data) only when an airdrop is in draft status (returns 400 VALIDATION_ERROR otherwise).
• Registers INVALID_STATE_TRANSITION (HTTP 409) in AppError and handles non-cancellable transitions on POST /api/v1/airdrops/:id/cancel.
• Updates documentation in README.md and adds comprehensive unit & integration tests covering state transitions, error codes, and cancellation logic.

Validation

• Ran full test suite via Jest (test/airdrops-service.test.js, test/airdrops.test.js, test/airdropExpiry.test.js, test/errorCodes.test.js, etc.).
• All 129 tests in affected suites passed cleanly.