Feat - Implement ticket reservation confirmation flow

[Alex0jk]

Summary

Implements the ticket reservation confirmation feature using a ticket-row state model (no separate reservation entity), plus endpoint contracts, tests, and a runnable API validation script.

What changed

• Added reservation state fields on tickets (reservation_status, reservation_created_at, reservation_expires_at, reservation_confirmed_at).
• Implemented reservation domain logic in service layer:
  • POST /api/tickets/<ticket_id>/reservation
  • POST /api/tickets/<ticket_id>/confirm
  • Runtime expiration checks (no persisted EXPIRED status)
  • Idempotent confirmation (200 when already confirmed)
  • Deterministic error contract (error_code, message, details)
• Added RESERVATION_TTL_SECONDS config default (60).
• Added API test-case script: scripts/api_test_cases.sh.
• Added integration tests for the reservation/confirm flow.
• Updated project docs to reference easy startup commands via run.sh.
• Added design session record for this feature.

API behavior

• Reserve endpoint returns:
  • 201 on tentative reservation
  • 409 when already active tentative reservation
  • 409 when already confirmed
  • 404 when ticket not found
• Confirm endpoint returns:
  • 200 on success
  • 200 when already confirmed (idempotent)
  • 409 when no active tentative reservation / expired at runtime
  • 404 when ticket not found

Validation performed

• Dockerized app startup verified via run.sh (--docker-detached).
• Health endpoint verified: GET /api/health.
• End-to-end API flow verified with:
  • ./scripts/api_test_cases.sh
  • Result: all checks passed.

Notes

• This PR follows the approved design decisions for single-reservation-per-ticket semantics and runtime expiration evaluation.