'Linear' shows up as an assignee — deactivate the leftover Linear bot user

A "Linear" user appears in the assignee picker. It's a leftover bot app_user from before the Linear ejection (PAR-118) — listUsers returns any app_user with active = true, and that row was never cleaned up.

Fix: Flyway migration V28 to deactivate it (active = false). listUsers already filters on active, so it drops out of the pickers immediately; issue.assignee_id is on delete set null and active is just a soft flag, so no references break and it's reversible. Targets the literal name "Linear" (the linear_id column that could have identified it precisely was dropped in V16).