Add jail distribution chart (Stage 5)

- backend: GET /api/dashboard/bans/by-jail endpoint
  - JailBanCount + BansByJailResponse Pydantic models in ban.py
  - bans_by_jail() service function with origin filter support
  - Route added to dashboard router
  - 17 new tests (7 service, 10 router); full suite 497 passed, 83% coverage

- frontend: JailDistributionChart component
  - JailBanCount / BansByJailResponse types in types/ban.ts
  - dashboardBansByJail endpoint constant in api/endpoints.ts
  - fetchBansByJail() in api/dashboard.ts
  - useJailDistribution hook in hooks/useJailDistribution.ts
  - JailDistributionChart component (horizontal bar chart, Recharts)
  - DashboardPage: full-width Jail Distribution section below Top Countries

frontend/src/pages/DashboardPage.tsx

@@ -19,6 +19,7 @@ import {
 } from "@fluentui/react-components";
 import { BanTable } from "../components/BanTable";
 import { BanTrendChart } from "../components/BanTrendChart";
+import { JailDistributionChart } from "../components/JailDistributionChart";
 import { ServerStatusBar } from "../components/ServerStatusBar";
 import { TopCountriesBarChart } from "../components/TopCountriesBarChart";
 import { TopCountriesPieChart } from "../components/TopCountriesPieChart";
@@ -166,6 +167,20 @@ export function DashboardPage(): React.JSX.Element {
         </div>
       </div>
 
+      {/* ------------------------------------------------------------------ */}
+      {/* Jail Distribution section                                           */}
+      {/* ------------------------------------------------------------------ */}
+      <div className={styles.section}>
+        <div className={styles.sectionHeader}>
+          <Text as="h2" size={500} weight="semibold">
+            Jail Distribution
+          </Text>
+        </div>
+        <div className={styles.tabContent}>
+          <JailDistributionChart timeRange={timeRange} origin={originFilter} />
+        </div>
+      </div>
+
       {/* ------------------------------------------------------------------ */}
       {/* Ban list section                                                    */}
       {/* ------------------------------------------------------------------ */}