feat: auto-reload grid when background cache refresh completes

Poll /api/cache-ts every 30s; silently reload concerts if the
cache timestamp increased (background scrape finished).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.py

@@ -90,6 +90,12 @@ async def api_concerts(page: int = 1, search: str = "", page_size: int = 24, cat
     return await fetch_concerts(page=page, search=search, page_size=page_size, category=category)
 
 
+@app.get("/api/cache-ts")
+async def api_cache_ts():
+    from arte_api import _cache
+    return {"ts": _cache["ts"], "count": len(_cache["data"])}
+
+
 @app.post("/api/refresh")
 async def api_refresh():
     count = await invalidate_cache()