Use has_many :through over HABTM

Always use `has_many :through` instead of `has_and_belongs_to_many`:
- Allows adding attributes to the join model
- Provides a model for the join table (validations, callbacks)
- More flexible for future requirements
- Easier to query and understand

```ruby
# Good
class User < ApplicationRecord
has_many :memberships
has_many :teams, through: :memberships
end

# Avoid
class User < ApplicationRecord
has_and_belongs_to_many :teams
end
```