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/hooks/useJailDistribution.ts

@@ -0,0 +1,85 @@
+/**
+ * `useJailDistribution` hook.
+ *
+ * Fetches per-jail ban counts for the jail distribution chart.
+ * Re-fetches automatically when `timeRange` or `origin` changes.
+ */
+
+import { useCallback, useEffect, useRef, useState } from "react";
+import { fetchBansByJail } from "../api/dashboard";
+import type { BanOriginFilter, JailBanCount, TimeRange } from "../types/ban";
+
+// ---------------------------------------------------------------------------
+// Return type
+// ---------------------------------------------------------------------------
+
+/** Return value shape for {@link useJailDistribution}. */
+export interface UseJailDistributionResult {
+  /** Jails ordered by ban count descending. */
+  jails: JailBanCount[];
+  /** Total ban count for the selected window. */
+  total: number;
+  /** True while a fetch is in flight. */
+  isLoading: boolean;
+  /** Error message or `null`. */
+  error: string | null;
+}
+
+// ---------------------------------------------------------------------------
+// Hook
+// ---------------------------------------------------------------------------
+
+/**
+ * Fetch and expose per-jail ban counts for the `JailDistributionChart` component.
+ *
+ * @param timeRange - Time-range preset: `"24h"`, `"7d"`, `"30d"`, or `"365d"`.
+ * @param origin    - Origin filter: `"all"`, `"blocklist"`, or `"selfblock"`.
+ * @returns Jail list, total count, loading state, and error.
+ */
+export function useJailDistribution(
+  timeRange: TimeRange,
+  origin: BanOriginFilter,
+): UseJailDistributionResult {
+  const [jails, setJails] = useState<JailBanCount[]>([]);
+  const [total, setTotal] = useState<number>(0);
+  const [isLoading, setIsLoading] = useState<boolean>(true);
+  const [error, setError] = useState<string | null>(null);
+
+  const abortRef = useRef<AbortController | null>(null);
+
+  const load = useCallback((): void => {
+    abortRef.current?.abort();
+    const controller = new AbortController();
+    abortRef.current = controller;
+
+    setIsLoading(true);
+    setError(null);
+
+    fetchBansByJail(timeRange, origin)
+      .then((data) => {
+        if (controller.signal.aborted) return;
+        setJails(data.jails);
+        setTotal(data.total);
+      })
+      .catch((err: unknown) => {
+        if (controller.signal.aborted) return;
+        setError(
+          err instanceof Error ? err.message : "Failed to fetch jail distribution",
+        );
+      })
+      .finally(() => {
+        if (!controller.signal.aborted) {
+          setIsLoading(false);
+        }
+      });
+  }, [timeRange, origin]);
+
+  useEffect(() => {
+    load();
+    return (): void => {
+      abortRef.current?.abort();
+    };
+  }, [load]);
+
+  return { jails, total, isLoading, error };
+}