Consolidation pass: HR-only access, effective-dated mutations, data integrity guards, test suite

Reworks the app from a two-role (hr_admin/manager) model to a single
HR-only role gated by profiles.is_active, fixes transfer/promote/karenz/
reorg RPCs to actually defer future-dated changes via a new
pending_org_changes table instead of writing them immediately (applied
by a daily Vercel Cron route), makes reorg undo append-only instead of
deleting history, adds Karenz-return and history-date integrity guards,
deprecates the salary column, and adds explicit schema grants + perf
indexes needed to run against a fresh (non-hosted) Postgres instance.

Adds vitest unit + integration test suites (the latter against a real
local Supabase instance) covering all of the above, plus lint/typecheck/
build wiring (`npm run check`).

app/(app)/employees/page.tsx

@@ -36,7 +36,7 @@ export default async function EmployeesPage({ searchParams }: EmployeesPageProps
   const to = from + PAGE_SIZE - 1;
 
   let query = supabase
-    .from("employees_directory")
+    .from("employees")
     .select(
       "id, first_name, last_name, personnel_number, job_title, team_id, division_id, location_id, entry_date, employment_type, weekly_hours, status",
       { count: "exact" }