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>

tests/unit/test_electrum_client.py

@@ -0,0 +1,77 @@
+import asyncio
+import json
+
+from app.electrum.client import ElectrumClient, ElectrumError
+
+
+class FakeWriter:
+    def __init__(self):
+        self.written = b""
+
+    def write(self, data: bytes) -> None:
+        self.written += data
+
+    async def drain(self) -> None:
+        pass
+
+    def close(self) -> None:
+        pass
+
+    async def wait_closed(self) -> None:
+        pass
+
+
+async def _client_with_fake_transport() -> tuple[ElectrumClient, asyncio.StreamReader, FakeWriter]:
+    client = ElectrumClient("localhost", 1234)
+    reader = asyncio.StreamReader()
+    writer = FakeWriter()
+    client._reader = reader
+    client._writer = writer
+    client._read_task = asyncio.create_task(client._read_loop())
+    return client, reader, writer
+
+
+async def test_request_resolves_on_matching_response():
+    client, reader, writer = await _client_with_fake_transport()
+
+    task = asyncio.create_task(client.request("blockchain.headers.subscribe"))
+    await asyncio.sleep(0)  # let request() write the payload
+    sent = json.loads(writer.written.decode())
+    assert sent["method"] == "blockchain.headers.subscribe"
+
+    reader.feed_data((json.dumps({"id": sent["id"], "result": {"height": 100}}) + "\n").encode())
+    result = await task
+    assert result == {"height": 100}
+
+    client._read_task.cancel()
+
+
+async def test_error_response_raises_electrum_error():
+    client, reader, writer = await _client_with_fake_transport()
+
+    task = asyncio.create_task(client.request("blockchain.transaction.broadcast", ["deadbeef"]))
+    await asyncio.sleep(0)
+    sent = json.loads(writer.written.decode())
+
+    reader.feed_data((json.dumps({"id": sent["id"], "error": "bad tx"}) + "\n").encode())
+    try:
+        await task
+        assert False, "expected ElectrumError"
+    except ElectrumError:
+        pass
+
+    client._read_task.cancel()
+
+
+async def test_notification_delivered_to_subscription_queue():
+    client, reader, writer = await _client_with_fake_transport()
+    queue = client.notifications("blockchain.scripthash.subscribe")
+
+    push = {"method": "blockchain.scripthash.subscribe", "params": ["abcd", "newstatus"]}
+    reader.feed_data((json.dumps(push) + "\n").encode())
+    await asyncio.sleep(0)
+
+    params = await asyncio.wait_for(queue.get(), timeout=1)
+    assert params == ["abcd", "newstatus"]
+
+    client._read_task.cancel()