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__/attendance.test.ts

@@ -258,6 +258,106 @@ describe("GET /api/admin/attendance (complete month, no truncation)", () => {
   });
 });
 
+describe("GET /api/admin/attendance (month window boundaries, @db.Date)", () => {
+  it("includes the month's own LAST day and excludes the neighbor month's first day", async () => {
+    // shift_date is @db.Date — Prisma compares the filter Dates by their UTC
+    // date part. The old LOCAL-midnight month bounds (22:00/23:00 UTC of the
+    // previous day) shifted the whole window a day back: the June list
+    // included May 31 and DROPPED June 30. Fixture rows sit exactly on the
+    // 2098-06 / 2098-07 boundary.
+    const juneLast = await prisma.attendance.create({
+      data: {
+        user_id: adminUserId,
+        shift_date: new Date(2098, 5, 30, 12, 0, 0), // 2098-06-30, local noon
+        leave_type: "work",
+        arrival_time: new Date(2098, 5, 30, 8, 0, 0),
+        departure_time: new Date(2098, 5, 30, 16, 0, 0),
+        notes: "attendance_wire_test june-last-day",
+      },
+    });
+    const julyFirst = await prisma.attendance.create({
+      data: {
+        user_id: adminUserId,
+        shift_date: new Date(2098, 6, 1, 12, 0, 0), // 2098-07-01, local noon
+        leave_type: "work",
+        arrival_time: new Date(2098, 6, 1, 8, 0, 0),
+        departure_time: new Date(2098, 6, 1, 16, 0, 0),
+        notes: "attendance_wire_test july-first-day",
+      },
+    });
+
+    const june = await app.inject({
+      method: "GET",
+      url: "/api/admin/attendance?year=2098&month=6&limit=100",
+      headers: { Authorization: `Bearer ${adminToken}` },
+    });
+    expect(june.statusCode).toBe(200);
+    const juneIds = june.json().data.map((r: { id: number }) => r.id);
+    expect(juneIds).toContain(juneLast.id);
+    expect(juneIds).not.toContain(julyFirst.id);
+
+    const july = await app.inject({
+      method: "GET",
+      url: "/api/admin/attendance?year=2098&month=7&limit=100",
+      headers: { Authorization: `Bearer ${adminToken}` },
+    });
+    expect(july.statusCode).toBe(200);
+    const julyIds = july.json().data.map((r: { id: number }) => r.id);
+    expect(julyIds).toContain(julyFirst.id);
+    expect(julyIds).not.toContain(juneLast.id);
+  });
+});
+
+describe("POST /api/admin/attendance (same-day duplicate window, @db.Date)", () => {
+  it("rejects a second leave record on the SAME day", async () => {
+    const first = await authPost("/api/admin/attendance", adminToken, {
+      user_id: adminUserId,
+      shift_date: "2098-08-10",
+      leave_type: "vacation",
+      leave_hours: 8,
+      notes: "attendance_wire_test dup-leave-first",
+    });
+    expect(first.statusCode).toBe(201);
+
+    // Old bug: the duplicate/overlap window was built from LOCAL midnights of
+    // the UTC-midnight shiftDate, so the validation queried ONLY the previous
+    // day — a same-day duplicate leave sailed through undetected.
+    const second = await authPost("/api/admin/attendance", adminToken, {
+      user_id: adminUserId,
+      shift_date: "2098-08-10",
+      leave_type: "sick",
+      leave_hours: 8,
+      notes: "attendance_wire_test dup-leave-second",
+    });
+    expect(second.statusCode).toBe(400);
+    expect(second.json().success).toBe(false);
+  });
+
+  it("does NOT falsely reject a leave on the day AFTER an existing record", async () => {
+    const first = await authPost("/api/admin/attendance", adminToken, {
+      user_id: adminUserId,
+      shift_date: "2098-08-10",
+      leave_type: "vacation",
+      leave_hours: 8,
+      notes: "attendance_wire_test neighbor-day-first",
+    });
+    expect(first.statusCode).toBe(201);
+
+    // Old bug (the other half): creating a record for the NEXT day queried
+    // the window [Aug 10, Aug 11) — i.e. yesterday's records — and bounced
+    // with a false "záznam o nepřítomnosti již existuje".
+    const nextDay = await authPost("/api/admin/attendance", adminToken, {
+      user_id: adminUserId,
+      shift_date: "2098-08-11",
+      leave_type: "vacation",
+      leave_hours: 8,
+      notes: "attendance_wire_test neighbor-day-next",
+    });
+    expect(nextDay.statusCode).toBe(201);
+    expect(nextDay.json().success).toBe(true);
+  });
+});
+
 describe("PUT /api/admin/attendance/:id (combined datetimes)", () => {
   it("updates a record with combined datetimes (incl. overnight departure)", async () => {
     const created = await prisma.attendance.create({