fix(salary): recompute entitled_days on existing ledger rows and record pre-cutover taken days (#1403)

The vacation ledger sync carried entitled_days verbatim on existing open
rows while re-deriving accrued and taken, so a stale entitled value (for
example the flat 25 stored before Semesterlagen 7 § pro-rating existed)
survived every sync. The recompute loop now re-derives entitled the same
way the lazy-seed path does, with the opening-balance cutover still
outranking recomputation for the year containing cutover_date.

Opening balances could also not record paid vacation days already taken
in the cutover year under the previous payroll system. New additive
column employee_opening_balances.vacation_days_taken_this_year (NUMERIC
NOT NULL DEFAULT 0, CHECK 0..40) threaded through the shared service,
the Zod schema, the MCP staging tool (schema + mergeable fields), the
staged-operation executor, the v1 REST routes, and the employee editor
form. Ledger semantics for the cutover year, on both seed and recompute
paths: entitled = remaining + taken_this_year, taken = booked-run taken
+ taken_this_year, so remaining keeps meaning remaining and the seeded
value survives every subsequent sync.

Fixes #1347

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

supabase/migrations/20260804120000_employee_opening_balances_taken_days.sql

@@ -0,0 +1,23 @@
+-- Migration: employee_opening_balances.vacation_days_taken_this_year
+-- (issue #1347: opening balances could not record already-taken days).
+--
+-- A company switching to Accounted mid-year may have paid vacation days
+-- already taken in the current vacation year under the previous payroll
+-- system. Those days are invisible to the ledger sync, which re-derives
+-- taken_days purely from BOOKED Accounted runs, so the cutover-year row
+-- understated both entitled and taken.
+--
+-- Ledger semantics for the vacation year containing cutover_date:
+--   entitled_days = vacation_paid_days_remaining + vacation_days_taken_this_year
+--   taken_days    = taken in booked runs + vacation_days_taken_this_year
+-- vacation_paid_days_remaining keeps meaning "remaining at cutover"
+-- (backward compatible: the public v1 REST API already exposes it).
+--
+-- The column rides the existing enforce_opening_balances_lock trigger:
+-- editable until the employee appears in a booked salary run.
+
+ALTER TABLE public.employee_opening_balances
+  ADD COLUMN vacation_days_taken_this_year NUMERIC NOT NULL DEFAULT 0
+    CHECK (vacation_days_taken_this_year >= 0 AND vacation_days_taken_this_year <= 40);
+
+NOTIFY pgrst, 'reload schema';