Add username field to the user data model as the foundation for
the multi-user permissions system (PLAN-407, TASK-408).
- SQLite migration 029 and Postgres migration 009 add username column
with partial unique index (WHERE username != '')
- User, UserCreate, UserUpdate Go structs updated
- Store: CreateUser, UpdateUser, scanUser, userColumns updated
- New GetUserByUsername store method (case-insensitive lookup)
- All auth handler JSON payloads include username field
- WorkspaceMember struct and ListWorkspaceMembers query include username
- TypeScript User type and API client inline types updated
Column is empty string by default; TASK-482 will backfill existing
users and TASK-409 will add validation/registration flow support.
xarmian
f80876a52e