AUTH-029: Soft delete for users and organizations

[lbedner]

Priority: Low | Estimate: M

Deletes are permanent with no recovery. No audit trail before deletion.

Scope

• Add deleted_at timestamp field to User and Organization models
• Soft delete sets timestamp instead of removing row
• Queries filter out soft-deleted records
• Optional: hard delete endpoint for permanent removal

Acceptance Criteria

• Delete operations set deleted_at instead of removing
• Queries exclude soft-deleted records
• Hard delete available for permanent removal
• Tests cover soft and hard delete