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/__tests__/invoices.service.test.ts

@@ -1,10 +1,12 @@
-import { describe, it, expect, afterEach } from "vitest";
+import { describe, it, expect, afterEach, beforeEach } from "vitest";
 import { Prisma } from "@prisma/client";
 import prisma from "../config/database";
 import {
   invoiceTotalWithVat,
   createInvoice,
   updateInvoice,
+  listInvoices,
+  getInvoiceListTotals,
 } from "../services/invoices.service";
 import { UpdateInvoiceSchema } from "../schemas/invoices.schema";
 
@@ -179,3 +181,83 @@ describe("updateInvoice — billing_text round-trips through UpdateInvoiceSchema
     expect(row?.billing_text).toBeNull();
   });
 });
+
+/* -------------------------------------------------------------------------- */
+/* Month filter boundaries — issue_date is @db.Date                           */
+/* -------------------------------------------------------------------------- */
+
+// issue_date is @db.Date: Prisma compares a Date filter by its UTC date part.
+// The old LOCAL-midnight month bounds (= 22:00/23:00 UTC of the previous day)
+// shifted the window a day back — the list/totals included the previous
+// month's last day and DROPPED invoices issued on the selected month's LAST
+// day. buildInvoiceWhere is shared by listInvoices AND getInvoiceListTotals,
+// so both are exercised. Fixtures use the test-only "TST" currency + far-future
+// 2098 dates so totals can be asserted exactly without colliding with other
+// suites' rows.
+
+describe("listInvoices / getInvoiceListTotals — month filter (@db.Date boundaries)", () => {
+  const cleanup = async () => {
+    // invoice_items cascade on invoice delete.
+    await prisma.invoices.deleteMany({ where: { currency: "TST" } });
+  };
+
+  beforeEach(cleanup);
+  afterEach(cleanup);
+
+  const listParams = {
+    page: 1,
+    limit: 100,
+    skip: 0,
+    sort: "id",
+    order: "asc" as const,
+    search: "",
+  };
+
+  async function createBoundaryFixtures() {
+    // Drafts: no invoice number is consumed, but buildInvoiceWhere has no
+    // status filter so they appear in the list/totals like any invoice.
+    const lastDayJan = await createInvoice({
+      status: "draft",
+      issue_date: "2098-01-31",
+      currency: "TST",
+      vat_rate: 21,
+      items: [{ quantity: 1, unit_price: 100, vat_rate: 21 }], // total 121
+    });
+    const firstDayFeb = await createInvoice({
+      status: "draft",
+      issue_date: "2098-02-01",
+      currency: "TST",
+      vat_rate: 21,
+      items: [{ quantity: 1, unit_price: 200, vat_rate: 21 }], // total 242
+    });
+    return { lastDayJan, firstDayFeb };
+  }
+
+  it("a month's list contains its own LAST day and not the neighbor month's first day", async () => {
+    const { lastDayJan, firstDayFeb } = await createBoundaryFixtures();
+
+    const jan = await listInvoices({ ...listParams, month: 1, year: 2098 });
+    const janIds = jan.data.map((i) => i.id);
+    expect(janIds).toContain(lastDayJan.id);
+    expect(janIds).not.toContain(firstDayFeb.id);
+
+    const feb = await listInvoices({ ...listParams, month: 2, year: 2098 });
+    const febIds = feb.data.map((i) => i.id);
+    expect(febIds).toContain(firstDayFeb.id);
+    expect(febIds).not.toContain(lastDayJan.id);
+  });
+
+  it("per-currency totals include the month's last day exactly once", async () => {
+    await createBoundaryFixtures();
+
+    const jan = await getInvoiceListTotals({ month: 1, year: 2098 });
+    const janTst = jan.totals.find((t) => t.currency === "TST");
+    // 100 + 21 % VAT — proves the Jan 31 invoice is counted and the
+    // Feb 1 invoice (242) is NOT pulled into January.
+    expect(janTst?.amount).toBe(121);
+
+    const feb = await getInvoiceListTotals({ month: 2, year: 2098 });
+    const febTst = feb.totals.find((t) => t.currency === "TST");
+    expect(febTst?.amount).toBe(242);
+  });
+});