feat: split scan config and app settings into separate endpoints

- New GET/POST /api/v1/settings for status check interval
- Scan /api/v1/scan/config now handles ranges only
- Frontend: settingsApi client, SettingsPanel uses settingsApi
- ScanConfigModal no longer reads/writes interval
- 4 new backend tests for settings endpoint

backend/tests/test_settings.py

@@ -0,0 +1,47 @@
+"""Tests for GET/POST /api/v1/settings."""
+from unittest.mock import patch
+
+import pytest
+from httpx import AsyncClient
+
+
+@pytest.fixture
+async def headers(client: AsyncClient):
+    res = await client.post("/api/v1/auth/login", json={"username": "admin", "password": "admin"})
+    token = res.json()["access_token"]
+    return {"Authorization": f"Bearer {token}"}
+
+
+@pytest.mark.asyncio
+async def test_get_settings_requires_auth(client: AsyncClient):
+    res = await client.get("/api/v1/settings")
+    assert res.status_code == 401
+
+
+@pytest.mark.asyncio
+async def test_get_settings_returns_interval(client: AsyncClient, headers):
+    res = await client.get("/api/v1/settings", headers=headers)
+    assert res.status_code == 200
+    data = res.json()
+    assert "interval_seconds" in data
+    assert isinstance(data["interval_seconds"], int)
+
+
+@pytest.mark.asyncio
+async def test_update_settings_saves_interval(client: AsyncClient, headers):
+    with patch("app.api.routes.settings.settings") as mock_settings:
+        mock_settings.status_checker_interval = 60
+        mock_settings.save_overrides = lambda: None
+        res = await client.post(
+            "/api/v1/settings",
+            json={"interval_seconds": 120},
+            headers=headers,
+        )
+    assert res.status_code == 200
+    assert res.json()["interval_seconds"] == 120
+
+
+@pytest.mark.asyncio
+async def test_update_settings_requires_auth(client: AsyncClient):
+    res = await client.post("/api/v1/settings", json={"interval_seconds": 30})
+    assert res.status_code == 401