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/main.py

@@ -6,6 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.api.routes import auth, canvas, edges, liveview, nodes, scan, status
+from app.api.routes import settings as settings_routes
 from app.core.config import settings
 from app.core.scheduler import start_scheduler, stop_scheduler
 from app.db.database import init_db
@@ -40,6 +41,7 @@ app.include_router(edges.router, prefix="/api/v1/edges", tags=["edges"])
 app.include_router(canvas.router, prefix="/api/v1/canvas", tags=["canvas"])
 app.include_router(scan.router, prefix="/api/v1/scan", tags=["scan"])
 app.include_router(status.router, prefix="/api/v1/status", tags=["status"])
+app.include_router(settings_routes.router, prefix="/api/v1/settings", tags=["settings"])
 app.include_router(liveview.router, prefix="/api/v1/liveview", tags=["liveview"])