Add demo data fixtures, fix deploy migrations, register admin models

[emidiovaleretto]

What

Three related housekeeping changes bundled in this branch:

1. Demo data fixtures (fixtures/demo_data.json)

Adds a seed fixture with a demo account and realistic sample data so the live API is testable without manual setup:

• demo_pilot user (login: demo_pilot / demo_pilot)
• 4 aircraft (A320, A21N, B738, B77W)
• 5 checklists + 9 checklist items across flight phases
• 6 flights on real routes (EIDW↔EGLL, LEBL, LFPG, EHAM, LPPT)
• 2 flight sessions

Load with: python manage.py loaddata fixtures/demo_data.json

2. Deploy migrations fix (railway.json)

The build uses the Dockerfile builder, so the Procfile's release: phase was being ignored and migrations never ran on deploy (production DB was left without tables). Adds preDeployCommand: python manage.py migrate so migrations run automatically on every deploy.

3. Register models in admin (checklist/admin.py, logbook/admin.py)

Registers the remaining models so they're manageable from the Django admin.

Notes

• loaddata is a one-time manual seed (run once against production), not part of the deploy — the automated step only runs migrate, never loaddata, to avoid overwriting live data.
• The fixture's demo_pilot is a regular user (no staff/superuser), so the demo account is contained.
• Because loaddata bypasses model save(), Flight.duration stays null (expected). The demo user's profile is created via signal / get_or_create after load.

How to test locally

1. python manage.py migrate
2. python manage.py loaddata fixtures/demo_data.json
3. Open /api/docs/, Authorize with demo_pilot / demo_pilot, and hit GET /api/flights/.