test: close coverage gaps hidden by seam mocking

Audit of the heavy-mock suites (scanner/status_checker/scheduler)
confirmed the mocking is sound layered seam-mocking, not brittle
internal patching. It did hide real gaps where a primitive was always
mocked and never exercised:

- status_checker._http_get: add status-code interpretation tests
  (2xx/4xx online, 5xx offline) + service-check exception -> offline.
  Module coverage 95% -> 100%.
- scheduler.reschedule_status_checks / reschedule_service_checks were
  wholly untested; add validation + guard tests. Add _run_proxmox_sync
  happy-path and error-swallow tests. Module coverage 76% -> 91%.

639 passed.

ha-relevant: no

backend/tests/test_status_checker.py

@@ -4,6 +4,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 
 from app.services.status_checker import (
+    _http_get,
     _ping,
     _tcp_connect,
     check_node,
@@ -11,6 +12,18 @@ from app.services.status_checker import (
     check_services,
 )
 
+
+def _mock_httpx_client(status_code):
+    """Build a stand-in for httpx.AsyncClient whose GET returns status_code."""
+    resp = MagicMock()
+    resp.status_code = status_code
+    client = MagicMock()
+    client.get = AsyncMock(return_value=resp)
+    ctx = MagicMock()
+    ctx.__aenter__ = AsyncMock(return_value=client)
+    ctx.__aexit__ = AsyncMock(return_value=False)
+    return MagicMock(return_value=ctx)
+
 # --- check_node dispatcher ---
 
 @pytest.mark.asyncio
@@ -462,3 +475,35 @@ async def test_check_services_returns_status_per_service():
 @pytest.mark.asyncio
 async def test_check_services_empty_list():
     assert await check_services("10.0.0.1", []) == []
+
+
+# --- _http_get status-code interpretation (real primitive, mocked transport) ---
+
+@pytest.mark.asyncio
+async def test_http_get_true_on_2xx():
+    with patch("app.services.status_checker.httpx.AsyncClient", _mock_httpx_client(200)):
+        assert await _http_get("http://host") is True
+
+
+@pytest.mark.asyncio
+async def test_http_get_true_on_4xx():
+    # A 4xx means the server is up and answering — still "online".
+    with patch("app.services.status_checker.httpx.AsyncClient", _mock_httpx_client(404)):
+        assert await _http_get("http://host") is True
+
+
+@pytest.mark.asyncio
+async def test_http_get_false_on_5xx():
+    with patch("app.services.status_checker.httpx.AsyncClient", _mock_httpx_client(503)):
+        assert await _http_get("http://host") is False
+
+
+@pytest.mark.asyncio
+async def test_check_service_http_exception_returns_offline():
+    svc = {"port": 80, "protocol": "tcp", "service_name": "http"}
+    with patch(
+        "app.services.status_checker._http_get",
+        new_callable=AsyncMock,
+        side_effect=RuntimeError("connection refused"),
+    ):
+        assert await check_service(svc, "10.0.0.1") == "offline"