Bug

Same issue as #201 but for the 402 paywall middleware. When --pay-chains is enabled, the 402 handler only checks generic sat balance, so users with only chain-specific deposits (e.g. tbtc4) get blocked even though they have funds.

Steps to reproduce

1. Start JSS with --pay --pay-chains tbtc3,tbtc4
2. Deposit a tbtc4 TXO → tbtc4: 2083219, sat: 0
3. GET /pay/secret.json with NIP-98 auth → 402 Payment Required (balance: 0)

Expected

The 402 middleware should fall back to chain-specific balances if generic sat is insufficient. Either:

• Accept a X-Pay-Currency: tbtc4 header to specify which balance to use
• Automatically try chain balances when sat balance is 0
• Return available balances in the 402 response so the client can retry with a currency header

Related

• Buy endpoint should accept chain-specific balances #201 — same fix applied to .buy endpoint
• Allow buy endpoint to accept chain-specific balances #202 — PR that fixed .buy