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/api/dashboard.ts

@@ -7,8 +7,9 @@
 import { get } from "./client";
 import { ENDPOINTS } from "./endpoints";
 import type {
-  BanTrendResponse,
   BanOriginFilter,
+  BansByJailResponse,
+  BanTrendResponse,
   DashboardBanListResponse,
   TimeRange,
 } from "../types/ban";
@@ -72,3 +73,23 @@ export async function fetchBanTrend(
   }
   return get<BanTrendResponse>(`${ENDPOINTS.dashboardBansTrend}?${params.toString()}`);
 }
+
+/**
+ * Fetch ban counts aggregated by jail for the selected time window.
+ *
+ * @param range  - Time-range preset: `"24h"`, `"7d"`, `"30d"`, or `"365d"`.
+ * @param origin - Origin filter: `"blocklist"`, `"selfblock"`, or `"all"`
+ *   (default `"all"`, which omits the parameter entirely).
+ * @returns {@link BansByJailResponse} with jails sorted by ban count descending.
+ * @throws {ApiError} When the server returns a non-2xx status.
+ */
+export async function fetchBansByJail(
+  range: TimeRange,
+  origin: BanOriginFilter = "all",
+): Promise<BansByJailResponse> {
+  const params = new URLSearchParams({ range });
+  if (origin !== "all") {
+    params.set("origin", origin);
+  }
+  return get<BansByJailResponse>(`${ENDPOINTS.dashboardBansByJail}?${params.toString()}`);
+}

frontend/src/api/endpoints.ts

@@ -31,6 +31,7 @@ export const ENDPOINTS = {
   dashboardBans: "/dashboard/bans",
   dashboardBansByCountry: "/dashboard/bans/by-country",
   dashboardBansTrend: "/dashboard/bans/trend",
+  dashboardBansByJail: "/dashboard/bans/by-jail",
 
   // -------------------------------------------------------------------------
   // Jails