Buy & use reinforcements

Characters earn money and can spend it on reinforcements

New entities:
- [x] Add reinforcement entity (faction, unit, cost)
- [x] Add credit journal entry entity(contains all income and expenses)
- [x] Add unit transactions entity (contains all buy&use transactions)

New API route:
- [x] Get the current units in stock (via elide)
- [ ] Select the units for the next battle

New validation:
- [ ] Add validation: you can't buy more units than you have current credits
- [ ] Add validation: you can't send more units than you have in stock