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

backend/app/routers/dashboard.py

@@ -5,8 +5,9 @@ fail2ban server health snapshot.  The snapshot is maintained by the
 background health-check task and refreshed every 30 seconds.
 
 Also provides ``GET /api/dashboard/bans`` for the dashboard ban-list table,
-``GET /api/dashboard/bans/by-country`` for country aggregation, and
-``GET /api/dashboard/bans/trend`` for time-bucketed ban counts.
+``GET /api/dashboard/bans/by-country`` for country aggregation,
+``GET /api/dashboard/bans/trend`` for time-bucketed ban counts, and
+``GET /api/dashboard/bans/by-jail`` for per-jail ban counts.
 """
 
 from __future__ import annotations
@@ -22,6 +23,7 @@ from app.dependencies import AuthDep
 from app.models.ban import (
     BanOrigin,
     BansByCountryResponse,
+    BansByJailResponse,
     BanTrendResponse,
     DashboardBanListResponse,
     TimeRange,
@@ -206,3 +208,39 @@ async def get_ban_trend(
     socket_path: str = request.app.state.settings.fail2ban_socket
 
     return await ban_service.ban_trend(socket_path, range, origin=origin)
+
+
+@router.get(
+    "/bans/by-jail",
+    response_model=BansByJailResponse,
+    summary="Return ban counts aggregated by jail",
+)
+async def get_bans_by_jail(
+    request: Request,
+    _auth: AuthDep,
+    range: TimeRange = Query(default=_DEFAULT_RANGE, description="Time-range preset."),
+    origin: BanOrigin | None = Query(
+        default=None,
+        description="Filter by ban origin: 'blocklist' or 'selfblock'. Omit for all.",
+    ),
+) -> BansByJailResponse:
+    """Return ban counts grouped by jail name for the selected time window.
+
+    Queries the fail2ban database and returns a list of jails sorted by
+    ban count descending.  This endpoint is intended for the dashboard jail
+    distribution bar chart.
+
+    Args:
+        request: The incoming request (used to access ``app.state``).
+        _auth: Validated session dependency.
+        range: Time-range preset — ``"24h"``, ``"7d"``, ``"30d"``, or
+            ``"365d"``.
+        origin: Optional filter by ban origin.
+
+    Returns:
+        :class:`~app.models.ban.BansByJailResponse` with per-jail counts
+        sorted descending and the total for the selected window.
+    """
+    socket_path: str = request.app.state.settings.fail2ban_socket
+
+    return await ban_service.bans_by_jail(socket_path, range, origin=origin)