feat: per-service status checks with offline colouring

Adds optional live status checking per service (not just per node),
requested as a follow-up to issue #196.

Backend:
- New check_service / check_services: HTTP(S) GET for web services, TCP
  connect otherwise; UDP and port-less non-web services stay 'unknown'.
- New scheduler job 'service_checks', independent interval (default 300s),
  added/removed live via set_service_checks_enabled.
- Settings gain service_check_enabled + service_check_interval (>=30s),
  persisted to scan_config.json. New WS message type 'service_status'.

Frontend:
- Live per-service status overlay in canvasStore (not persisted, so it
  never round-trips through canvas save), fed by the WS message.
- DetailPanel + canvas node service rows: offline service turns red
  (#f85149), otherwise keeps its category colour.
- SettingsModal: toggle + interval input (default 300s / 5 min).

Off by default — no behaviour change until enabled.

ha-relevant: yes

backend/tests/test_scheduler.py

@@ -5,7 +5,13 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 
-from app.core.scheduler import _run_status_checks, start_scheduler, stop_scheduler
+from app.core.scheduler import (
+    _run_service_checks,
+    _run_status_checks,
+    set_service_checks_enabled,
+    start_scheduler,
+    stop_scheduler,
+)
 from app.db.database import Base
 from app.db.models import Node
 
@@ -141,6 +147,7 @@ def test_scheduler_uses_settings_interval():
     with patch("app.core.scheduler.settings") as mock_settings, \
          patch("app.core.scheduler.AsyncIOScheduler", return_value=mock_sched):
         mock_settings.status_checker_interval = 45
+        mock_settings.service_check_enabled = False
         start_scheduler()
         _, kwargs = mock_sched.add_job.call_args
         assert kwargs["seconds"] == 45
@@ -155,3 +162,90 @@ def test_start_and_stop_scheduler():
         mock_sched.add_job.assert_called_once()
         mock_sched.start.assert_called_once()
         mock_sched.shutdown.assert_called_once()
+
+
+# ---------------------------------------------------------------------------
+# Service checks
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_run_service_checks_disabled_does_nothing(mem_db):
+    async with mem_db() as session:
+        session.add(_make_node(services=[{"port": 80, "protocol": "tcp", "service_name": "http"}]))
+        await session.commit()
+
+    with patch("app.core.scheduler.settings") as mock_settings, \
+         patch("app.core.scheduler.AsyncSessionLocal", mem_db), \
+         patch("app.services.status_checker.check_services", new_callable=AsyncMock) as mock_cs:
+        mock_settings.service_check_enabled = False
+        await _run_service_checks()
+    mock_cs.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_run_service_checks_broadcasts_per_node(mem_db):
+    async with mem_db() as session:
+        node = _make_node(
+            ip="10.0.0.5",
+            services=[{"port": 80, "protocol": "tcp", "service_name": "http"}],
+        )
+        session.add(node)
+        await session.commit()
+        node_id = node.id
+
+    statuses = [{"port": 80, "protocol": "tcp", "status": "offline"}]
+
+    with patch("app.core.scheduler.settings") as mock_settings, \
+         patch("app.core.scheduler.AsyncSessionLocal", mem_db), \
+         patch("app.core.scheduler.check_services", new_callable=AsyncMock, return_value=statuses), \
+         patch("app.api.routes.status.broadcast_service_status", new_callable=AsyncMock) as mock_bcast:
+        mock_settings.service_check_enabled = True
+        await _run_service_checks()
+
+    mock_bcast.assert_awaited_once()
+    _, kwargs = mock_bcast.call_args
+    assert kwargs["node_id"] == node_id
+    assert kwargs["services"] == statuses
+
+
+@pytest.mark.asyncio
+async def test_run_service_checks_skips_nodes_without_services(mem_db):
+    async with mem_db() as session:
+        session.add(_make_node(ip="10.0.0.6", services=[]))
+        await session.commit()
+
+    with patch("app.core.scheduler.settings") as mock_settings, \
+         patch("app.core.scheduler.AsyncSessionLocal", mem_db), \
+         patch("app.core.scheduler.check_services", new_callable=AsyncMock) as mock_cs:
+        mock_settings.service_check_enabled = True
+        await _run_service_checks()
+    mock_cs.assert_not_called()
+
+
+def test_set_service_checks_enabled_adds_and_removes_job():
+    mock_sched = MagicMock()
+    mock_sched.running = True
+    with patch("app.core.scheduler.scheduler", mock_sched), \
+         patch("app.core.scheduler.settings") as mock_settings:
+        mock_settings.service_check_interval = 300
+        # Enable: no existing job -> add
+        mock_sched.get_job.return_value = None
+        set_service_checks_enabled(True)
+        mock_sched.add_job.assert_called_once()
+        # Disable: existing job -> remove
+        mock_sched.get_job.return_value = MagicMock()
+        set_service_checks_enabled(False)
+        mock_sched.remove_job.assert_called_once_with("service_checks")
+
+
+def test_start_scheduler_adds_service_job_when_enabled():
+    mock_sched = MagicMock()
+    with patch("app.core.scheduler.settings") as mock_settings, \
+         patch("app.core.scheduler.AsyncIOScheduler", return_value=mock_sched):
+        mock_settings.status_checker_interval = 60
+        mock_settings.service_check_enabled = True
+        mock_settings.service_check_interval = 300
+        start_scheduler()
+    job_ids = [kw.get("id") for _, kw in mock_sched.add_job.call_args_list]
+    assert "status_checks" in job_ids
+    assert "service_checks" in job_ids