Replace BTCPay payments with free pre-orders + live ready-status

- server.js: order store with K484-XXXX codes, stand assignment, SSE
  status to customers, passcode-gated staff API, persists to .data/
- staff.html at /staff: order board with Mark ready / Collected / Cancel
- index.html: drop BTCPay API key, invoices, QR, BTC price; orders now
  server-backed with live PREPARING -> READY updates and stand location
- e2e-test.js rewritten for the order loop (14 checks, all passing)
- README added; .gitignore covers server/.data

server/package.json

@@ -1,13 +1,12 @@
 {
-  "name": "kitchen484-btc-pay",
-  "version": "1.0.0",
+  "name": "kitchen484-server",
+  "version": "2.0.0",
   "private": true,
-  "description": "BTCPay proxy backend for KITCHEN 484 / SOLARPUNK SUMMIT — keeps the BTCPay API key server-side",
+  "description": "KITCHEN 484 / SOLARPUNK SUMMIT \u2014 pre-order + ready-status backend",
   "type": "module",
   "main": "server.js",
   "scripts": {
     "start": "node server.js",
-    "mock": "node mock-btcpay.js",
     "test:e2e": "node e2e-test.js"
   },
   "engines": {