Webledger deposit addresses: phased design #267

Overview

Give each webledger a master keypair so users can send sats to a standard Bitcoin address instead of passing TXO URIs with private keys. Built in phases alongside the existing deposit flow.

Phase 1: Single pod address

• Webledger gets a master keypair (generated on first --pay start, stored in .well-known/webledgers/)
• GET /pay/.address returns the pod's taproot deposit address
• User sends sats from any wallet, then claims: POST /pay/.deposit with {"txid": "...", "vout": 0}
• Pod verifies on mempool API, credits the authenticated user's balance
• No private key in the URL — pod owns the key

Phase 2: Per-user tweaked addresses

• GET /pay/.address?user=did:nostr:abc... returns a unique address: P2TR(masterKey + SHA256(userDID))
• Pod can map incoming transactions to users automatically
• Enables deposits without a claim step

Phase 3: Auto-detection

• Pod polls mempool API for known addresses
• Credits balances automatically on confirmation
• Address gap management for tweaked addresses

Notes

• Existing TXO voucher deposit flow (txo:chain:txid:vout?key=) remains unchanged
• Phase 1 reuses existing mempool verification from deposit handler
• Key derivation primitives already exist in src/token.js
• Pod becomes custodial (holds keys) — same as token trail already does