Add PHPStan, switch to PHPUnit, update for API SDK resources

[sandervanhooft]

Breaking Changes ⚠️

Namespace Migration

Import contracts from Laravel namespace (recommended) or Fluent namespace:

// Before
use Vatly\Contracts\BillableInterface;

// After (recommended for Laravel users)
use Vatly\Laravel\Contracts\BillableInterface;

// Or use Fluent directly
use Vatly\Fluent\Contracts\BillableInterface;

Interface Changes (inherited from vatly-fluent-php)

• BillableInterface::getKey() return type: string|int → mixed
• BillableInterface::save() return type: void → mixed
• BillableInterface::getMorphClass() removed

New Laravel Wrapper Interfaces

• Vatly\Laravel\Contracts\BillableInterface extends Fluent interface
• Allows Laravel users to import only from Vatly\Laravel namespace

Other Changes

• Switched test framework from Pest to PHPUnit
• Added PHPStan with strict type checking
• Updated models to work with API SDK resources directly

Migration Guide

1. Update use statements to new namespace
2. If implementing BillableInterface directly, update method signatures
3. Requires vatly/vatly-fluent-php from same release

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Part of Larafast dev sprint.