feat: add QR code event check-in system

Backend:
- Add event_checkin_tokens table with unique tokens per event
- Implement GET /api/events/:slug/details endpoint (on-demand token generation)
- Implement POST /api/events/checkin/:token endpoint (date validation only in production)
- Implement DELETE /api/events/:slug/leave endpoint
- Add comprehensive test suite for check-in endpoints

Frontend:
- Add EventDetailsPage with QR code display, participant list, and stats
- Add EventCheckinPage with success/error screens
- Add "Leave Event" button with confirmation modal to EventChatPage
- Install qrcode.react library for QR code generation
- Update routing and API client with new endpoints

Features:
- QR codes valid from (startDate-1d) to (endDate+1d)
- Development mode bypasses date validation for testing
- Automatic participant count tracking
- Duplicate check-in prevention
- Token reuse for same event (generated once, cached)

backend/prisma/migrations/20251114125544_add_event_checkin_tokens/migration.sql

@@ -0,0 +1,18 @@
+-- CreateTable
+CREATE TABLE "event_checkin_tokens" (
+    "id" SERIAL NOT NULL,
+    "event_id" INTEGER NOT NULL,
+    "token" VARCHAR(50) NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "event_checkin_tokens_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "event_checkin_tokens_event_id_key" ON "event_checkin_tokens"("event_id");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "event_checkin_tokens_token_key" ON "event_checkin_tokens"("token");
+
+-- AddForeignKey
+ALTER TABLE "event_checkin_tokens" ADD CONSTRAINT "event_checkin_tokens_event_id_fkey" FOREIGN KEY ("event_id") REFERENCES "events"("id") ON DELETE CASCADE ON UPDATE CASCADE;