feat: add reports center, admin drill-down, disease filter + bug fixes + perf optimization

Frontend features:
- 报表中心 (ReportsCenter): list/detail views, diagnosis breakdown chart, CSV export
- 多级行政下钻 (AdminBreadcrumb): 湖北省→武汉市→区→街道 hierarchical drill-down
- 按病种筛选 (DiseaseFilter): multi-select diagnosis filter on monitoring + reports pages

Backend:
- Add /forecast/{days} endpoint, diagnosis filter params on cases endpoints
- Add /streets aggregation endpoint, enrich reports with real case data
- Extract shared case_loader module

Bug fixes (14):
- Fix missing /risk/forecast route (404), historyApi pointing to non-existent router
- Fix min_risk filter silently ignored in insights/hotspots
- Fix type mismatches: CaseTrendResponse, CaseStatsResponse shapes
- Fix silent .catch(() => {}) swallowing errors, fetchAlerts not clearing stale state
- Fix lru_cache caching exceptions, generateReport used cachedGet for write op
- Fix missing useEffect deps in Insights, DistrictComparison, ReportsCenter

Performance (9):
- Zustand selectors across 9 components (eliminate re-render cascades)
- Fix districtCases.sort() mutating store state, inline IIFE → memo'd component
- CaseLocationMap: React.memo, race protection, correct deps
- AlertCard: stable callbacks, TimelinePlayer: useMemo, TopNav: clock isolation
- SideNav: modules array to module scope, DiseaseFilter: memoized filter

backend/routers/grid.py

@@ -23,10 +23,14 @@ from models import (
 router = APIRouter(prefix="/api", tags=["grid"])
 
 
-@lru_cache(maxsize=1)
+_parquet_cache: dict[str, "pd.DataFrame"] = {}
+
 def _load_parquet(path: Path) -> "pd.DataFrame":
     import pandas as pd
-    return pd.read_parquet(path)
+    key = str(path)
+    if key not in _parquet_cache:
+        _parquet_cache[key] = pd.read_parquet(path)
+    return _parquet_cache[key]
 
 
 @router.get("/history/aggregated", response_model=HistoricalAggregationResponse)