V100: Staff module tables.

Creates four tables used by phoenix_kit_staff:

• phoenix_kit_staff_departments — top-level org units
• phoenix_kit_staff_teams — teams inside a department
• phoenix_kit_staff_people — staff profiles, each linked 1:1 to a phoenix_kit_users row (required FK)
• phoenix_kit_staff_team_memberships — join table for team membership

UUIDv7 primary keys, timestamptz timestamps, cascading deletes department → team → team_memberships; person deletion cascades to team_memberships; user deletion cascades to the staff person profile.

Departments and teams are identified by UUID only — no slug columns. A team's name must be unique within its department.