Add read-only admin impersonation

Admins (with 2FA) can view the site as a non-admin user for up to 30
minutes, with a required reason and a "Viewing as" banner. It is
strictly read-only: a guard mounted before every route refuses any
write made with an impersonation token, protect/optionalAuth check the
session is live, and GET handlers skip the price-refresh writes. Start,
end, reason and every page/API call (including blocked attempts) are
audited; optional email to the user.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CP62L9JxHomA36aYP1ekCe

backend/prisma/migrations/20261019181608_add_impersonation/migration.sql

@@ -0,0 +1,55 @@
+-- AlterEnum
+-- This migration adds more than one value to an enum.
+-- With PostgreSQL versions 11 and earlier, this is not possible
+-- in a single migration. This can be worked around by creating
+-- multiple migrations, each migration adding only one value to
+-- the enum.
+
+
+ALTER TYPE "AdminAuditAction" ADD VALUE 'impersonation_started';
+ALTER TYPE "AdminAuditAction" ADD VALUE 'impersonation_ended';
+
+-- CreateTable
+CREATE TABLE "ImpersonationSession" (
+    "id" TEXT NOT NULL,
+    "adminId" TEXT NOT NULL,
+    "targetUserId" TEXT NOT NULL,
+    "reason" TEXT NOT NULL,
+    "startedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "expiresAt" TIMESTAMP(3) NOT NULL,
+    "endedAt" TIMESTAMP(3),
+    "endedReason" TEXT,
+
+    CONSTRAINT "ImpersonationSession_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "ImpersonationRequestLog" (
+    "id" TEXT NOT NULL,
+    "sessionId" TEXT NOT NULL,
+    "method" TEXT NOT NULL,
+    "path" TEXT NOT NULL,
+    "blocked" BOOLEAN NOT NULL DEFAULT false,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "ImpersonationRequestLog_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "ImpersonationSession_adminId_idx" ON "ImpersonationSession"("adminId");
+
+-- CreateIndex
+CREATE INDEX "ImpersonationSession_targetUserId_idx" ON "ImpersonationSession"("targetUserId");
+
+-- CreateIndex
+CREATE INDEX "ImpersonationRequestLog_sessionId_createdAt_idx" ON "ImpersonationRequestLog"("sessionId", "createdAt");
+
+-- AddForeignKey
+ALTER TABLE "ImpersonationSession" ADD CONSTRAINT "ImpersonationSession_adminId_fkey" FOREIGN KEY ("adminId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ImpersonationSession" ADD CONSTRAINT "ImpersonationSession_targetUserId_fkey" FOREIGN KEY ("targetUserId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ImpersonationRequestLog" ADD CONSTRAINT "ImpersonationRequestLog_sessionId_fkey" FOREIGN KEY ("sessionId") REFERENCES "ImpersonationSession"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+

backend/prisma/schema.prisma

@@ -111,6 +111,8 @@ model User {
   ledGroups        AllocationGroup[]   @relation("AllocationGroupLeader")
   feedbackInvites  FeedbackInvite[]
   feedbackGiven    FeedbackResponse[]
+  impersonationsDone ImpersonationSession[] @relation("ImpersonationAdmin")
+  impersonatedIn     ImpersonationSession[] @relation("ImpersonationTarget")
   scanLocksForced  ScanLock[]          @relation("ScanLockForcedBy")
   syncReportsDone  OfflineSyncReport[] @relation("SyncReportReviewer")
 
@@ -499,6 +501,8 @@ enum AdminAuditAction {
   scan_lock_force_released
   offline_sync
   ticket_transferred
+  impersonation_started
+  impersonation_ended
 }
 
 // Append-only audit trail for admin/supervisor-initiated actions with money or
@@ -854,6 +858,37 @@ enum TicketDelivery {
 
 // One named person on a group registration, for one main-ticket place. Gets their
 // own quantity-1 ticket and their own attendee form response.
+/// An admin viewing the site as another user (strictly read-only, at most 30 minutes).
+model ImpersonationSession {
+  id           String                    @id @default(uuid())
+  adminId      String
+  targetUserId String
+  reason       String
+  startedAt    DateTime                  @default(now())
+  expiresAt    DateTime
+  endedAt      DateTime?
+  endedReason  String? // 'ended' | 'expired'
+  admin        User                      @relation("ImpersonationAdmin", fields: [adminId], references: [id], onDelete: Cascade)
+  target       User                      @relation("ImpersonationTarget", fields: [targetUserId], references: [id], onDelete: Cascade)
+  requests     ImpersonationRequestLog[]
+
+  @@index([adminId])
+  @@index([targetUserId])
+}
+
+/// Every page viewed and API call made during an impersonation session.
+model ImpersonationRequestLog {
+  id        String               @id @default(uuid())
+  sessionId String
+  method    String
+  path      String
+  blocked   Boolean              @default(false)
+  createdAt DateTime             @default(now())
+  session   ImpersonationSession @relation(fields: [sessionId], references: [id], onDelete: Cascade)
+
+  @@index([sessionId, createdAt])
+}
+
 /// First-party funnel tracking (only after cookie consent): one row per anonymous
 /// session, event and stage ('view' | 'start' | 'submit'). "Paid" comes from the
 /// registration that carries the session id. Rows older than 90 days are rolled up