Register Eloquent class alias for phpstan

[firecow]

Summary

• Registers the Eloquent class alias via bootstrapFiles so phpstan always knows the class exists
• The \Eloquent alias is normally registered during Laravel bootstrap, but when that fails intermittently in CI, phpstan reports class.notFound for @mixin \Eloquent in model files
• bootstrapFiles is the correct phpstan mechanism for making classes discoverable (stubs only override PHPDocs, they don't register new classes)

Test plan

• Verify phpstan no longer reports class.notFound for @mixin \Eloquent