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>

frontend/src/components/clinical/DonutChart.tsx

@@ -0,0 +1,56 @@
+import { memo } from 'react';
+import { PieChart, Pie, Cell, Tooltip, Legend, ResponsiveContainer } from 'recharts';
+import { CLINICAL_COLORS, TOOLTIP_STYLE } from './chartColors';
+
+export interface DonutSlice {
+  name: string;
+  value: number;
+}
+
+interface DonutChartProps {
+  data: DonutSlice[];
+  /** name -> color。未命中时按 palette 顺序回退。 */
+  colorMap?: Record<string, string>;
+}
+
+/** 通用环形图。复用于「出院结局构成」与「入院途径构成」。 */
+export const DonutChart = memo(function DonutChart({ data, colorMap }: DonutChartProps) {
+  if (!data || data.length === 0) {
+    return <div className="text-center py-8 text-text-muted text-sm">暂无数据</div>;
+  }
+
+  const total = data.reduce((s, d) => s + d.value, 0);
+  const colorFor = (name: string, idx: number) =>
+    colorMap?.[name] ??
+    CLINICAL_COLORS.routePalette[idx % CLINICAL_COLORS.routePalette.length] ??
+    CLINICAL_COLORS.outcomeFallback;
+
+  return (
+    <ResponsiveContainer width="100%" height={280}>
+      <PieChart>
+        <Pie
+          data={data}
+          dataKey="value"
+          nameKey="name"
+          cx="50%"
+          cy="50%"
+          innerRadius={56}
+          outerRadius={88}
+          paddingAngle={2}
+        >
+          {data.map((d, idx) => (
+            <Cell key={d.name} fill={colorFor(d.name, idx)} />
+          ))}
+        </Pie>
+        <Tooltip
+          contentStyle={TOOLTIP_STYLE}
+          formatter={(v: number, name: string) => [
+            `${v.toLocaleString()}（${total > 0 ? ((v / total) * 100).toFixed(1) : '0'}%）`,
+            name,
+          ]}
+        />
+        <Legend wrapperStyle={{ fontSize: '11px' }} />
+      </PieChart>
+    </ResponsiveContainer>
+  );
+});