Add CSV import/export for students (Stage 11)

Import wizard (4 steps):
- Upload CSV with UTF-8 or Windows-1251 (iconv-lite) decoding
- Auto-detect columns: Email, Имя, Фамилия, Телефон
- Preview table with per-row status: new / update / error
- Options: auto-verify email, assign course + access days, send welcome email
- Apply: creates users with bcrypt password + Account record, grants enrollments

Export:
- GET /api/admin/export-users with course filter + encoding selection
- UTF-8 with BOM (works in all apps) or Windows-1251 (legacy Excel)
- Fields: Email, Имя, Телефон, Дата регистрации, Курсы, Прогресс

Navigation: added "Импорт / Экспорт" link to admin sidebar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/api/admin/export-users/route.ts

@@ -0,0 +1,64 @@
+import { NextRequest, NextResponse } from "next/server";
+import { headers } from "next/headers";
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
+import iconv from "iconv-lite";
+
+export async function GET(request: NextRequest) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session || session.user.role !== "admin") {
+    return NextResponse.json({ error: "Нет доступа" }, { status: 403 });
+  }
+
+  const { searchParams } = request.nextUrl;
+  const courseId = searchParams.get("courseId") || undefined;
+  const encoding = (searchParams.get("encoding") as "utf8" | "win1251") ?? "utf8";
+
+  // Fetch users
+  const users = await prisma.user.findMany({
+    where: courseId
+      ? { enrollments: { some: { courseId } } }
+      : { role: "student" },
+    orderBy: { createdAt: "desc" },
+    include: {
+      enrollments: {
+        include: { course: { select: { title: true } } },
+      },
+      progress: { select: { lessonId: true } },
+    },
+  });
+
+  // Build CSV rows
+  const csvHeaders = ["Email", "Имя", "Телефон", "Дата регистрации", "Курсы", "Прогресс (уроков)"];
+  const rows = users.map((u) => {
+    const courses = u.enrollments.map((e) => e.course.title).join(" | ");
+    const progress = u.progress.length;
+    const registeredAt = new Date(u.createdAt).toLocaleDateString("ru-RU");
+    return [u.email, u.name, "", registeredAt, courses, String(progress)];
+  });
+
+  const allRows = [csvHeaders, ...rows];
+  const csvText = allRows
+    .map((row) => row.map((cell) => `"${String(cell).replace(/"/g, '""')}"`).join(";"))
+    .join("\r\n");
+
+  // Encode
+  let body: Buffer;
+  let charset: string;
+  if (encoding === "win1251") {
+    body = iconv.encode(csvText, "win1251");
+    charset = "windows-1251";
+  } else {
+    body = Buffer.from("\uFEFF" + csvText, "utf8"); // BOM for Excel
+    charset = "utf-8";
+  }
+
+  const filename = `students_${new Date().toISOString().slice(0, 10)}.csv`;
+
+  return new NextResponse(body as unknown as BodyInit, {
+    headers: {
+      "Content-Type": `text/csv; charset=${charset}`,
+      "Content-Disposition": `attachment; filename="${filename}"`,
+    },
+  });
+}