Initial capay platform scaffold

app/.env

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+APP_PORT=3000
+APP_HOST=https://capay.hao.work
+PLAN=free
+
+# Low-cost polling for free plan
+TX_POLL_INTERVAL_MS=180000
+MAX_TX_CHECK_PER_CYCLE=20
+MIN_CONFIRMATIONS=1
+
+# Alchemy Ethereum RPC (supports multiple free keys)
+ALCHEMY_API_KEYS=P9kZiHB6Q7CLrBlMsUN3n
+ALCHEMY_API_KEY=P9kZiHB6Q7CLrBlMsUN3n
+ETHEREUM_RPC_URL=https://eth-mainnet.g.alchemy.com/v2/P9kZiHB6Q7CLrBlMsUN3n
+# Optional: comma-separated RPC URLs (overrides/extends)
+# ETHEREUM_RPC_URLS=https://eth-mainnet.g.alchemy.com/v2/KEY1,https://eth-mainnet.g.alchemy.com/v2/KEY2
+
+# Webhook signature secrets (replace in production)
+IPN_HMAC_SECRET=replace-with-strong-secret
+BTCPAY_WEBHOOK_SECRET=replace-with-strong-secret
+X402_WEBHOOK_SECRET=replace-with-strong-secret
+
+# Payment platform
+DB_PATH=./data/db.json
+WEBHOOK_TIMEOUT_MS=8000