fix(host-agent): daemon heartbeat + watchdog to recover stale channel subscription

[Pher217]

Goal

Fix the "worked last week, broken now" reverse-input failure: a user's Telegram keystrokes (`pty.inject`) silently stopped reaching tmux after the daemon had been connected for hours.

Root cause: the backend `HostDaemonConsumer`'s Redis channel-receive stalls silently (after Redis reconnect churn). The WebSocket stays `ESTABLISHED`, daphne logs no disconnect, but `group_send` messages are never delivered to the daemon. The daemon can't detect this at the ws level — the connection looks healthy, it just receives nothing. Restarting the daemon (fresh consumer + fresh Redis receive) fixes it, proving the connection was "deaf but alive".

Changes

• host-agent/agent_host/wsclient.py — per-connection liveness marker + two new coroutines in the existing gather:
  • _heartbeat: sends host_heartbeat up every HEARTBEAT_INTERVAL (30s).
  • _watchdog: if no ping returns within HEARTBEAT_TIMEOUT (90s), raises to tear down the connection so the existing outer loop reconnects with a fresh signed URL.
  • _receiver: a ping host_command updates the liveness marker.
• backend/apps/hostlink/consumers.py — receive_json handles host_heartbeat by group_send-ing a ping host_command back to the host's group. This deliberately round-trips through group_send → Redis → consumer.host_command → ws — the exact path pty.inject uses — so a stalled consumer fails the heartbeat and triggers recovery.

Approach

Heartbeat is echoed through the group path (not just the raw ws) so it tests the thing that actually decays. Watchdog reconnects only on timeout, so healthy connections are never disrupted. Degrades gracefully: if the backend never echoes, the watchdog becomes a ~90s periodic reconnect — still fixing the decay.

Test Summary

• Daemon (host-agent/tests/test_wsclient.py): 3 new tests — heartbeat sent periodically, ping resets liveness (no reconnect), watchdog forces reconnect on silence. 24 passed.
• Backend (backend/apps/hostlink/tests/test_consumers.py): 1 new test — host_heartbeat echoes a ping via the group. 8 passed.
• New tests pass isolated in 3.2s; reuse existing fake-ws/injectable-connect helpers. No auth/signing/send_keys/offline-queue code touched.

Notes

• Implemented by a delegated worker to a fixed spec (tasks/specs/daemon-heartbeat-durability.md), reviewed line-by-line.
• Pre-existing ruff import warnings (lines 27/196) and an asyncio teardown warning in older tests are unrelated and left untouched (surgical scope).

Files Changed

• host-agent/agent_host/wsclient.py — heartbeat + watchdog
• backend/apps/hostlink/consumers.py — heartbeat echo
• host-agent/tests/test_wsclient.py, backend/apps/hostlink/tests/test_consumers.py — tests
• tasks/specs/daemon-heartbeat-durability.md — spec