feat(ST-111ci8t4): admin user-actions (delete, updateRoles, setMustResetPassword)

- lib/session.ts: isAdmin: boolean toegevoegd aan SessionData
- lib/auth-guard.ts: requireAdmin() toegevoegd (redirect /dashboard bij !isAdmin)
- actions/admin/users.ts: deleteUserAction (zelfbescherming), updateUserRolesAction
  (Zod z.nativeEnum, eigen ADMIN-rol-beveiliging, transactie), setMustResetPasswordAction
  — alle drie 'use server', revalidatePath('/admin/users')

actions/admin/users.ts

@@ -0,0 +1,43 @@
+'use server'
+
+import { revalidatePath } from 'next/cache'
+import { z } from 'zod'
+import { Role } from '@prisma/client'
+import { prisma } from '@/lib/prisma'
+import { requireAdmin } from '@/lib/auth-guard'
+
+export async function deleteUserAction(userId: string) {
+  const session = await requireAdmin()
+  if (userId === session.userId) {
+    throw new Error('Zelfverwijdering niet toegestaan')
+  }
+  await prisma.user.delete({ where: { id: userId } })
+  revalidatePath('/admin/users')
+}
+
+const rolesSchema = z.array(z.nativeEnum(Role))
+
+export async function updateUserRolesAction(userId: string, roles: Role[]) {
+  const session = await requireAdmin()
+
+  const parsed = rolesSchema.safeParse(roles)
+  if (!parsed.success) {
+    throw new Error('Ongeldige rol-waarden')
+  }
+
+  if (userId === session.userId && !parsed.data.includes(Role.ADMIN)) {
+    throw new Error('Kan eigen ADMIN-rol niet verwijderen')
+  }
+
+  await prisma.$transaction([
+    prisma.userRole.deleteMany({ where: { user_id: userId } }),
+    ...parsed.data.map((role) => prisma.userRole.create({ data: { user_id: userId, role } })),
+  ])
+  revalidatePath('/admin/users')
+}
+
+export async function setMustResetPasswordAction(userId: string, value: boolean) {
+  await requireAdmin()
+  await prisma.user.update({ where: { id: userId }, data: { must_reset_password: value } })
+  revalidatePath('/admin/users')
+}