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/app/api/routes/settings.py

@@ -0,0 +1,29 @@
+"""App-level settings (status checker interval, etc.)."""
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+
+from app.api.deps import get_current_user
+from app.core.config import settings
+
+router = APIRouter()
+
+
+class AppSettings(BaseModel):
+    interval_seconds: int
+
+
+@router.get("", response_model=AppSettings)
+async def get_settings(_: str = Depends(get_current_user)) -> AppSettings:
+    return AppSettings(interval_seconds=settings.status_checker_interval)
+
+
+@router.post("", response_model=AppSettings)
+async def update_settings(
+    payload: AppSettings, _: str = Depends(get_current_user)
+) -> AppSettings:
+    try:
+        settings.status_checker_interval = payload.interval_seconds
+        settings.save_overrides()
+        return payload
+    except Exception as exc:
+        raise HTTPException(status_code=500, detail=str(exc)) from exc