fix: analysis 500s, caching, alert page perf

P0: Fix KeyError in 3 analysis endpoints. geojson.py stores 1d risk
as "risk_value" but analysis.py accessed "risk_1d" — always crashed.

Backend: Add lru_cache to GeoJSON/CSV/Parquet loaders, date helpers,
and district loader. Add try/except and FileNotFoundError guards.

Frontend: Debounce riskRange, merge counts into useMemo, stabilize
handleGridClick with ref, memoize nearest-grid scan, wrap AlertMap
in React.memo, switch useLodGrid from fetch to cachedGet.

backend/utils/date_helpers.py

@@ -3,6 +3,7 @@ Date utilities: finding latest dates from GeoJSON files, parsing date strings.
 """
 import glob
 import re
+from functools import lru_cache
 from pathlib import Path
 
 from fastapi import HTTPException
@@ -10,6 +11,7 @@ from fastapi import HTTPException
 from config import DATA_DIR, DATE_FORMAT_GEOJSON
 
 
+@lru_cache(maxsize=1)
 def get_latest_date() -> str:
     """Get latest available date from GeoJSON files in DATA_DIR."""
     pattern = str(DATA_DIR / "risk_*.geojson")
@@ -29,6 +31,7 @@ def get_latest_date() -> str:
     return max(dates)
 
 
+@lru_cache(maxsize=1)
 def get_available_dates(days: int = 30) -> list[str]:
     """Get list of available dates, most recent first."""
     pattern = str(DATA_DIR / "risk_*.geojson")