feat(users): split user name fields and add computed attributes

[ibourgeois]

Goal

Replace the single user name column with first and last names while preserving convenient derived accessors.

Scope

• Add a migration from name to first_name and last_name
• Add a computed name attribute
• Add a computed initials attribute
• Update factories and tests

Out of Scope

• Profile editing UI
• Team membership features

Acceptance Criteria

• User records store first and last names separately
• Existing code can access a computed name value where needed
• Avatar and display usage can access computed initials

Dependencies

• Depends on feat(auth): install Fortify and wire backend authentication flows #7

Test Plan

• Add migration coverage as appropriate
• Add model attribute tests for name and initials
• Update factory-driven tests for the new schema

[ibourgeois]

Duplicate of #7, which already merged the name split and computed accessors.