perf: stale-while-revalidate cache + startup pre-warm

If DB has any concerts data (even expired), return it immediately and
refresh in background. Start pre-warming at container startup so the
scrape runs before the first user request.

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

main.py

@@ -11,7 +11,7 @@ from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 from pydantic import BaseModel
 
-from arte_api import fetch_concerts, get_concerts_by_category, invalidate_cache, CATEGORIES
+from arte_api import fetch_concerts, get_all_concerts, get_concerts_by_category, invalidate_cache, CATEGORIES
 from downloader import DownloadManager
 
 logging.basicConfig(level=logging.INFO)
@@ -56,6 +56,7 @@ async def lifespan(app: FastAPI):
     tasks = [
         asyncio.create_task(dm.start_worker()),
         asyncio.create_task(_auto_dl_loop()),
+        asyncio.create_task(get_all_concerts()),   # pre-warm cache at startup
     ]
     yield
     for t in tasks: