Add comprehensive mixin documentation with timestamps and soft delete patterns

[Copilot]

The mixin documentation lacked practical examples for common patterns like timestamp tracking and soft deletes, particularly how to override lifecycle methods like unlink().

Documentation Changes (docs/mixins.md)

• Extension vs Composition: Clarified two mixin patterns - registering multiple classes with same static _name vs using static mixins = [...]

• Timestamps Mixin: Full Timestampable implementation using pre_create/pre_update hooks to auto-manage created_at/updated_at fields

• Soft Delete Mixin: Complete SoftDeletable pattern overriding unlink() to set deleted_at instead of deleting, with restore(), forceUnlink(), and default scope to hide deleted records

• Activity Tracking: Polymorphic relations example for generic activity logging

• Best Practices: Combining multiple mixins, conflict resolution, abstract models

Working Demo (demo/mixins/)

Runnable examples with:

• Timestampable and SoftDeletable mixin implementations
• User and Document models demonstrating composition
• Interactive script validating all features

class SoftDeletable {
  static abstract = true;
  static fields = { deleted_at: { type: 'datetime', default: null } };
  static defaultScope = { where: { deleted_at: null } };
  
  async unlink() {
    if (this.deleted_at) return await this.forceUnlink();
    await this.write({ deleted_at: new Date() });
    return this;
  }
  
  async restore() {
    await this.write({ deleted_at: null });
    return this;
  }
}

class Documents {
  static mixins = ['Timestampable', 'SoftDeletable'];
  static fields = { id: 'primary', title: 'string' };
}

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