Add Electrum SPV client and address listener

Minimal Electrum protocol client (client.py) plus a scripthash
subscription listener (listener.py) that watches user deposit addresses
for confirmed UTXOs, with the address->scripthash conversion helper and
a manual smoke-test script against the dev bootstrap server.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/electrum_smoke_test.py

@@ -0,0 +1,27 @@
+"""Manual smoke test: connect to the configured Electrum server, do the
+server.version handshake, subscribe to headers and confirm a sane tip height.
+
+Usage: PYTHONPATH=. python scripts/electrum_smoke_test.py
+"""
+
+import asyncio
+
+from app.config import settings
+from app.electrum.client import ElectrumClient
+
+
+async def main() -> None:
+    client = ElectrumClient(settings.electrum_host, settings.electrum_port, settings.electrum_use_ssl)
+    await client.connect()
+    print("connected + handshake OK")
+
+    header = await client.subscribe_headers()
+    print("tip:", header)
+    assert header["height"] > 0
+
+    await client.close()
+    print("OK")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())