## What
`StoreCategoryRequest` and `UpdateCategoryRequest` had identical 24-line
`prepareForValidation()` deriving `cashflow_direction` from category
type.
Moved to `app/Http/Requests/Concerns/ResolvesCategoryCashflowDirection`
trait.
## Stats
- **-48 / +43 lines** (net small, removes a whole duplicated logic block
that must stay in sync)
## Checks
- `php artisan test --filter=Categor` — 104 passed (565 assertions)
- `vendor/bin/pint --dirty` — pass
Part of duplication-removal series (#475–#478).
fe692e37c3