feat: add organizations and memberships

[vinimachadosantana]

What

Adds Organization and Membership models with roles, plus endpoints to list and create organisations.

Why

The tenancy boundary. Every resource from here belongs to an organisation, and membership is what grants access to it.

How

• role is an integer enum with compact storage, plus generated predicates and scopes.
• A composite unique index on [user_id, organization_id] stops duplicate memberships at the database level, where it can't be raced.
• Creating an organisation and its first membership happens in a transaction.
• index reads through current_user.organizations, so the association itself scopes the query.

Testing

• Creator becomes an admin
• Listing returns only the user's own organisations