feat: implement secure multi-merchant HODL invoice escrow and automated settlement system

[GautamBytes]

Secure Multi-Merchant HODL Escrow System

🚀Summary

This PR implements a robust, trust-minimized HODL Invoice system for Shopstr, enabling secure escrow-style payments for physical goods. It replaces the legacy "Direct Zap" flow with a conditional "Handshake" protocol (Order Request → HODL Invoice → Automated Settlement) using NIP-17 DMs and NIP-47 (NWC). This architecture ensures funds are only moved after inventory is verified by the merchant's automated agent.

Key Features

Secure HODL Handshake

• Implements the full NIP-17 handshake where buyer funds are locked in their own wallet via Alby's makeHoldInvoice.
• Funds are only settled once the merchant's backend bot confirms stock and verifies shipping details.

Multi-Merchant Cart Support

• Logic automatically groups products by merchant during checkout.
• Sends individual encrypted DMs to each seller and manages multiple simultaneous HODL payment loops in a single UI session.

Automated Settlement Bot

• A serverless-compatible background worker polls pending invoice states and settles funds once the "held" status is confirmed.
• Built-in network resilience: Uses a Retry Queue (failed_settlements table) to handle temporary relay or API downtime.

Transactional Integrity & Security

• Uses PostgreSQL FOR UPDATE row-level locking for inventory reservations to prevent double-booking.
• All merchant credentials (NWC strings and bot keys) are AES-encrypted server-side.
• Ephemeral bot keys are wiped from React memory immediately after syncing to the server.

🛠 Architectural Changes

• Database: Added hodl_invoices, inventory_reservations, and failed_settlements tables. Automatic schema migration is handled in db-service.ts.
• SDK: Upgraded to Alby SDK v7.0.0 to support HODL methods.
• APIs: Introduced:
  • /api/inventory/check (bulk pre-flight check)
  • /api/merchant/save-connection (secure key sync)
  • /api/settlement/run (CRON heartbeat)

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