Allow admins to be part of multiple organizations

[jeroenrinzema]

Goal

The admins table currently has an organization_id column, limiting admins to a single organization. This needs to be refactored so users can belong to multiple organizations.

Scope

Database Migration

• Create a new organization_users many-to-many table linking admins to organizations with roles (member, admin, owner).
• Backfill organization_users from existing admins.organization_id data.
• Drop the organization_id column from the admins table after backfill.

Backend Updates

• Update all backend queries that rely on admins.organization_id to use the new organization_users table.
• Update the Clerk auth flow — currently auto-provisions a single org on first login. Needs to handle users that already exist across multiple organizations.

OpenFGA Authorization Model

• Update the OpenFGA authorization model and relationship tuples to reflect the new organization_users structure.
• Ensure RBAC checks work correctly for users belonging to multiple organizations.