feat: implement Horizon SSE payment streaming with cursor persistence

[cokehill]

Summary

The stream manager should open a Horizon SSE connection per active payment subscription and
persist the paging cursor to SQLite so streams resume correctly after a restart.

Acceptance criteria

• StreamManager._streamPayments() opens an SSE stream via @stellar/stellar-sdk
• Cursor saved to stream_cursors table after each event
• On restart, stream resumes from saved cursor (not "now")
• Filter by asset_code and min_amount applied before queuing delivery
• Unit test: mock Horizon SSE, assert delivery is queued for matching events
• Unit test: assert filtered events (wrong asset) do NOT queue a delivery

Notes

Use server.payments().forAccount(addr).cursor(saved).stream({ onmessage }).
Cursor must be saved per account key, not globally.