feat: add organization entity schema

[xx7412421-cloud]

Closes #112.

Summary

• add an Organization model with owner tracking and a unique slug
• add OrganizationMember membership support with per-organization uniqueness
• add the generated migration for the organizations and organization_members tables, indexes, and foreign keys

Validation

• npm ci
• npm exec -- prisma validate
• npm run prisma:generate
• npm exec -- prisma format
• npm run build:backend
• git diff --check

[mijinummi]

Hi @xx7412421-cloud , thanks for your contribution. Kindly fix the database migration check ci that is failing. Wonderful contribution so far.

[xx7412421-cloud]

Thanks for the review. I pushed a follow-up to address the migration check failure:

• added a Prisma 7 prisma.config.ts so migrate commands can read DATABASE_URL
• updated the database migration workflow to deploy migrations before checking status, then generate the Prisma client

Local validation passed with:

• npx prisma validate
• npx prisma generate