Replace polling with WebSocket for real-time Signal message reception

Switch from polling /v1/receive every 2s to a persistent WebSocket
connection at ws://.../v1/receive/<number>. Messages now arrive
instantly via the signal-cli-rest-api WebSocket endpoint.

- Add `listen()` generator to SignalClient using websockets library
- Extract `_parse_envelope()` as standalone function
- Replace `run_loop` polling with WebSocket listener + reconnect logic
- Remove `poll_interval` from BotConfig and CLI args
- Add websockets to Nix overlay and pyproject.toml dependencies

https://claude.ai/code/session_01AKXQBuVBsW7J1YbukDiQ7A

pyproject.toml

@@ -21,6 +21,7 @@ dependencies = [
     "requests",
     "sqlalchemy",
     "typer",
+    "websockets",
 ]
 
 [project.scripts]