feat(organizations): migration script for existing users and data

[PierreBrisorgueil]

Description

Migration script that creates default organizations for all existing users and re-scopes their data.

Details

Uses the migration system (#3218). File: modules/organizations/migrations/2026-03-10-organizations-init.js

Steps:

1. For each existing user without a membership:
   • Create a default organization (name: "{user.firstName}'s organization", auto-generated slug)
   • Create a membership (userId, organizationId, role: owner)
   • Set user.currentOrganization = organizationId
2. For each existing task/upload without organizationId:
   • Set organizationId = owner's default organization
3. Idempotent: skip users who already have an organization

Acceptance criteria

• All existing users get a default organization + owner membership
• All existing tasks/uploads get organizationId
• Idempotent (safe to run multiple times)
• Runs automatically at boot via migration system