feat(export): emit ISO-8601 with user-tz offset instead of Z

v1.4.25 W7 phase 3.1 — closes the export half of issue #167.

CSV/JSON exports previously emitted timestamps via .toISOString(),
producing 2026-05-15T09:00:00.000Z. Excel and LibreOffice drop the
trailing Z silently and render the bare datetime in the viewer's
locale, so a Warsaw user reads UTC as local time — 11:00 entries
display as 09:00 in their spreadsheet.

Export helpers now accept an optional userTz argument and emit
ISO-8601 with the user's UTC offset
(2026-05-15T11:00:00+02:00). The offset survives the round-trip and
the on-screen value is unambiguous regardless of the spreadsheet's
defaults. Routes resolve the user's stored timezone via the v1.4.25
W7 phase 2 resolver and thread it through formatMeasurements /
formatIntakeEvents / formatMoodEntries.

Backward-compatible: userTz is optional. When omitted (admin tools,
canonical-backup-on-disk shape) the helpers fall back to
.toISOString(), keeping the legacy contract for the JSON full-backup
route that the upload-restore validator depends on.

Surfaces touched:
  - src/lib/export.ts (3 formatters + helper)
  - src/app/api/export/measurements/route.ts
  - src/app/api/export/medications/route.ts (intake history block)
  - src/app/api/export/mood/route.ts
  - src/app/api/export/route.ts (legacy aggregate route)

Tests: 5 new cases in src/lib/__tests__/export.test.ts covering
Berlin / Auckland / New_York offsets, the no-userTz fallback, and
the mood-entry case where the stored YYYY-MM-DD date column stays
unchanged (see proposal §6.3 for the read-side rationale).

src/app/api/export/measurements/route.ts

@@ -23,6 +23,7 @@ import { auditLog } from "@/lib/auth/audit";
 import { apiError, getClientIp } from "@/lib/api-response";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { toCSV, formatMeasurementsForExport } from "@/lib/export";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
 import { NextRequest, NextResponse } from "next/server";
 
 export const GET = apiHandler(async (request: NextRequest) => {
@@ -37,12 +38,15 @@ export const GET = apiHandler(async (request: NextRequest) => {
   const { since, until } = parseRange(request.url);
   const where = buildWhere(user.id, { since, until });
 
-  const measurements = await prisma.measurement.findMany({
-    where,
-    orderBy: { measuredAt: "desc" },
-  });
+  const [measurements, userTz] = await Promise.all([
+    prisma.measurement.findMany({
+      where,
+      orderBy: { measuredAt: "desc" },
+    }),
+    resolveUserTimezone(user.id),
+  ]);
 
-  const csv = toCSV(formatMeasurementsForExport(measurements));
+  const csv = toCSV(formatMeasurementsForExport(measurements, userTz));
 
   await auditLog("user.export.measurements", {
     userId: user.id,

src/app/api/export/medications/route.ts

@@ -24,6 +24,7 @@ import {
   formatMedicationsForExport,
   formatIntakeEventsForExport,
 } from "@/lib/export";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
 import { NextRequest, NextResponse } from "next/server";
 
 export const GET = apiHandler(async (request: NextRequest) => {
@@ -48,23 +49,26 @@ export const GET = apiHandler(async (request: NextRequest) => {
   // without a matching migration); a wildcard `include: { schedules: true }`
   // would 500 on the missing column. Pin the columns instead — the export
   // shape doesn't need every future field anyway.
-  const medications = await prisma.medication.findMany({
-    where: { userId: user.id },
-    select: {
-      name: true,
-      dose: true,
-      active: true,
-      schedules: {
-        select: {
-          windowStart: true,
-          windowEnd: true,
-          label: true,
-          dose: true,
+  const [medications, userTz] = await Promise.all([
+    prisma.medication.findMany({
+      where: { userId: user.id },
+      select: {
+        name: true,
+        dose: true,
+        active: true,
+        schedules: {
+          select: {
+            windowStart: true,
+            windowEnd: true,
+            label: true,
+            dose: true,
+          },
         },
       },
-    },
-    orderBy: { createdAt: "desc" },
-  });
+      orderBy: { createdAt: "desc" },
+    }),
+    resolveUserTimezone(user.id),
+  ]);
 
   const sections: string[] = [
     "# Medications",
@@ -91,7 +95,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
     });
     intakeCount = events.length;
     sections.push("# Intake history");
-    sections.push(toCSV(formatIntakeEventsForExport(events)));
+    sections.push(toCSV(formatIntakeEventsForExport(events, userTz)));
   }
 
   const body = sections.join("\n\n");

src/app/api/export/mood/route.ts

@@ -19,6 +19,7 @@ import { auditLog } from "@/lib/auth/audit";
 import { apiError, getClientIp } from "@/lib/api-response";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { toCSV, formatMoodEntriesForExport } from "@/lib/export";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
 import { NextRequest, NextResponse } from "next/server";
 
 export const GET = apiHandler(async (request: NextRequest) => {
@@ -50,12 +51,15 @@ export const GET = apiHandler(async (request: NextRequest) => {
     if (until) where.moodLoggedAt.lte = until;
   }
 
-  const entries = await prisma.moodEntry.findMany({
-    where,
-    orderBy: { moodLoggedAt: "desc" },
-  });
+  const [entries, userTz] = await Promise.all([
+    prisma.moodEntry.findMany({
+      where,
+      orderBy: { moodLoggedAt: "desc" },
+    }),
+    resolveUserTimezone(user.id),
+  ]);
 
-  const csv = toCSV(formatMoodEntriesForExport(entries));
+  const csv = toCSV(formatMoodEntriesForExport(entries, userTz));
 
   await auditLog("user.export.mood", {
     userId: user.id,

src/app/api/export/route.ts

@@ -11,6 +11,7 @@ import {
   formatIntakeEventsForExport,
   formatMoodEntriesForExport,
 } from "@/lib/export";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
 import { NextRequest, NextResponse } from "next/server";
 
 /**
@@ -48,6 +49,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
   }
 
   const userId = user.id;
+  const userTz = await resolveUserTimezone(userId);
 
   const data: Record<string, unknown> = {};
 
@@ -58,8 +60,8 @@ export const GET = apiHandler(async (request: NextRequest) => {
     });
     data.measurements =
       format === "csv"
-        ? toCSV(formatMeasurementsForExport(measurements))
-        : formatMeasurementsForExport(measurements);
+        ? toCSV(formatMeasurementsForExport(measurements, userTz))
+        : formatMeasurementsForExport(measurements, userTz);
   }
 
   if (type === "medications" || type === "all") {
@@ -82,8 +84,8 @@ export const GET = apiHandler(async (request: NextRequest) => {
     });
     data.intakeEvents =
       format === "csv"
-        ? toCSV(formatIntakeEventsForExport(events))
-        : formatIntakeEventsForExport(events);
+        ? toCSV(formatIntakeEventsForExport(events, userTz))
+        : formatIntakeEventsForExport(events, userTz);
   }
 
   if (type === "mood" || type === "all") {
@@ -93,8 +95,8 @@ export const GET = apiHandler(async (request: NextRequest) => {
     });
     data.moodEntries =
       format === "csv"
-        ? toCSV(formatMoodEntriesForExport(moodEntries))
-        : formatMoodEntriesForExport(moodEntries);
+        ? toCSV(formatMoodEntriesForExport(moodEntries, userTz))
+        : formatMoodEntriesForExport(moodEntries, userTz);
   }
 
   await auditLog("export.download", {