feat: deep statistical analytics — clinical, symptoms, incidence, env correlation, weekday

Adds a substantial layer of data-backed statistics (all grounded in verified,
clean source data — no fabricated metrics).

Backend (new routers/statistics.py, prefix /api/stats; +106 pytest still green):
- /inpatient-clinical: LOS dist + by-disease quartiles, cost dist + by-disease +
  cost-vs-LOS, outcome counts, admission-route counts, BMI-by-age, KPIs
  (5822 admissions, median LOS 4d, mean ¥6294, cure 99.1%, emergency 47%)
- /symptoms: 主诉 keyword frequencies (发热/咳嗽/肺炎…) + revisit ratio (36%)
- /incidence-rate: per-10k-population standardized rate by district (cases ÷ pop)
- /env-correlation: pollutant×cases Pearson + 7×7 pairwise matrix + PM2.5 scatter
- /temporal: weekday distribution (+ month/yoy returned but UI omits them — data
  is December-only, so seasonality/YoY would be misleading)

Frontend:
- NEW 住院临床分析 page (/analysis/clinical, nav 临床分析): 9 charts + KPI row —
  LOS histogram + box-by-disease, cost histogram + scatter + by-disease, outcome
  donut (severity-colored), admission-route donut, age-band BMI box
- DiseaseAnalysis: 主诉症状词频 horizontal bar + revisit ratio
- DistrictComparison: 标化发病率（每万人）with 病例数↔发病率 toggle (rate is
  epidemiologically correct; raw counts mislead by population)
- EnvironmentalHealth: pollutant-cases correlation bar + 7×7 correlation heatmap +
  PM2.5×cases scatter with least-squares regression line
- TrendAnalysis: 星期就诊分布 + honest "data is December-only" note
- statsApi client + types

Gates: tsc 0 · build ok · functional e2e 43/43 (incl 2 new clinical) · verified
live against real backend data via dev proxy

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -13,7 +13,7 @@ from logging_config import setup_logging
 from middleware.request_logger import RequestLoggerMiddleware
 from auth.router import router as auth_router
 from auth.service import seed_default_admin
-from routers import risk, alerts, analysis, insights, reports, cases, geocoded, grid, chat, environment
+from routers import risk, alerts, analysis, insights, reports, cases, geocoded, grid, chat, environment, statistics
 
 
 setup_logging()
@@ -59,6 +59,7 @@ app.include_router(geocoded.router)
 app.include_router(grid.router)
 app.include_router(chat.router)
 app.include_router(environment.router)
+app.include_router(statistics.router)
 
 
 @app.get("/")