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/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