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/types/ban.ts

@@ -110,3 +110,31 @@ export interface BanTrendResponse {
   /** Human-readable bucket size label, e.g. `"1h"`, `"6h"`, `"1d"`, `"7d"`. */
   bucket_size: string;
 }
+
+// ---------------------------------------------------------------------------
+// Bans by jail
+// ---------------------------------------------------------------------------
+
+/**
+ * A single jail entry in the bans-by-jail aggregation.
+ *
+ * Mirrors `JailBanCount` from `backend/app/models/ban.py`.
+ */
+export interface JailBanCount {
+  /** Jail name. */
+  jail: string;
+  /** Number of bans recorded in this jail for the selected window. */
+  count: number;
+}
+
+/**
+ * Response from `GET /api/dashboard/bans/by-jail`.
+ *
+ * Mirrors `BansByJailResponse` from `backend/app/models/ban.py`.
+ */
+export interface BansByJailResponse {
+  /** Jails ordered by ban count descending. */
+  jails: JailBanCount[];
+  /** Total ban count in the selected window. */
+  total: number;
+}