feat(entity-extension): add entity extension system

[michalbiarda]

Summary

• Any module can now add columns to an existing entity's table without touching the original entity class
• Extensions are auto-discovered from src/EntityExtension/ directories across vendor/modules/app at boot
• Hydrated transparently from the same DB row; accessed via $entity->extension(MyExtension::class) with full IDE type inference via @template

What was built

• EntityExtension abstract base class + #[ExtensionOf] attribute to declare which entity a class extends
• EntityExtensionRegistry singleton + EntityExtensionDiscovery scanner — registered and populated in module.php boot
• ExtensionMetadata + EntityExtensionMetadataFactory — parses extension classes; validates no #[Table], no primary key, no relationships
• EntityMetadata::$extensions field (backward-compatible last param) — EntityMetadataFactory merges extensions and detects column/property name conflicts loud at boot
• Entity::extension() / setExtension() — typed accessor with @template T of EntityExtension for IDE inference
• EntityHydrator — hydrates extension objects from the same DB row; silently skips extensions whose columns are absent (safe during rolling deploys)
• Repository — INSERT/UPDATE/batch include extension columns; null/default/loud-error policy enforced at save time
• SchemaBuilder — extension columns included in table schema for migration diffs
• packages/database/README.md updated with full developer workflow

Test plan

• 47 new tests added across 10 new/modified test files
• Full suite: 4956 passing

🤖 Generated with Claude Code

[michalbiarda]

@markshust This adds a basic Entity extensibility and could later be updated with more complex extensions (using joins, etc.).