fix(dates): @db.Date filters built from local midnight queried the wrong day

Prisma truncates a JS Date used in a WHERE filter on a @db.Date column to
its UTC date part. Under TZ=Europe/Prague a local-midnight boundary
(new Date(y,m,d) = 22:00/23:00Z of the previous day) therefore filtered as
the PREVIOUS calendar date. Same class as the dashboard fix; full sweep of
every @db.Date filter in the codebase. New shared helper
utcMidnightOfLocalDay() in src/utils/date.ts.

Fixed (all previously off by one day):
- attendance.service: getStatus today+month windows; getWorkfund (last day
  of each month was double-counted across months); getPrintData (monthly
  print included prev month's last day); listAttendance (admin month view
  included prev month's last day AND dropped the selected month's last
  day); createAttendance duplicate/overlap validation (checked only the
  PREVIOUS day - same-day duplicates were never caught, neighbors falsely
  rejected)
- invoice-alerts: the 'splatnost za 3 dny' advance alert had NEVER fired
  (due==today+3 was outside the fetched window); window computation
  extracted as computeAlertWindow() + pure tests
- invoices.service: month list/totals filter dropped invoices issued on the
  month's last day; getInvoiceStats month/year bounds; markOverdueInvoices
  boundary; auto paid_date could store yesterday during 00:00-02:00
- received-invoices auto paid_date, issued-orders default order_date: same
  night-window write hazard, now via the helper
- attendance.schema: shift_date hardened to isoDateString (bare YYYY-MM-DD)

+9 regression tests (month boundaries, same-day duplicate, last-day-of-month
invoice in list+totals, alert window).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/routes/admin/received-invoices.ts

@@ -15,6 +15,7 @@ import {
 } from "../../schemas/received-invoices.schema";
 import { nasInvoicesManager } from "../../services/nas-financials-manager";
 import { toCzk } from "../../services/exchange-rates";
+import { utcMidnightOfLocalDay } from "../../utils/date";
 import path from "path";
 
 const VALID_STATUSES = ["unpaid", "paid"] as const;
@@ -561,14 +562,17 @@ export default async function receivedInvoicesRoutes(
       // `amount` is the GROSS total (VAT included); VAT is the portion within it.
       const computedVat = vatFromGross(finalAmount, finalVatRate);
 
-      // Auto-set paid_date when status transitions to paid (matching PHP)
+      // Auto-set paid_date when status transitions to paid (matching PHP).
+      // paid_date is @db.Date (truncated to the UTC date part) —
+      // utcMidnightOfLocalDay keeps the LOCAL calendar day even during the
+      // 00:00–02:00 Prague window (a bare new Date() stored yesterday there).
       const newStatus =
         body.status !== undefined
           ? String(body.status)
           : String(existing.status);
       const paidDate =
         newStatus === "paid" && String(existing.status) !== "paid"
-          ? new Date()
+          ? utcMidnightOfLocalDay()
           : body.paid_date !== undefined
             ? body.paid_date
               ? new Date(String(body.paid_date))