feat(admin): add admin panel with user management, daily puzzle CRUD, and activity graph

- Add admin page at /admin (restricted to ADMIN_EMAIL via proxy)
- Implement user ban/unban and deletion
- Add daily puzzle creation, modification, deletion
- Include 14-day activity chart with hover details
- Add User.banned field to schema and migrate database
- Block banned users from logging in
- Add ADMIN_EMAIL to .env.local configuration
- Update Prisma client after schema changes

app/api/admin/users/[id]/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from "next/server";
+import { prisma } from "../../../../../lib/prisma";
+
+type Params = { params: Promise<{ id: string }> };
+
+// DELETE /api/admin/users/[id] — supprimer un utilisateur
+export async function DELETE(_req: Request, { params }: Params) {
+  const { id } = await params;
+  await prisma.user.delete({ where: { id } });
+  return NextResponse.json({ ok: true });
+}
+
+// PATCH /api/admin/users/[id] — ban/unban
+export async function PATCH(req: Request, { params }: Params) {
+  const { id } = await params;
+  const { banned } = await req.json() as { banned: boolean };
+  const user = await prisma.user.update({ where: { id }, data: { banned } });
+  return NextResponse.json({ id: user.id, banned: user.banned });
+}