refactor(requests): extract user-owned exists rules to trait (#477)
## What
The `Rule::exists('x', 'id')->where(fn => user_id)` ownership closure
was repeated **19×** across 10 Form Requests (categories, labels,
accounts, typed accounts).
New `app/Http/Requests/Concerns/ValidatesUserOwnedResources` trait:
- `$this->userOwned('categories' | 'labels' | 'accounts')`
- `$this->userOwnedAccountOfType(AccountType::Loan | RealEstate)`
`UpdateAutomationRuleRequest` intentionally untouched — #476 makes it
extend Store.
## Stats
- **-110 / +63 lines**
## Checks
- `php artisan test
--filter="Transaction|Budget|Account|AutomationRule|RealEstate|Loan"` —
641 passed (3052 assertions), 1 skipped
- `vendor/bin/pint --dirty` — pass
Part of duplication-removal series (#475, #476).
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