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/services/ban_service.py

@@ -24,10 +24,12 @@ from app.models.ban import (
     TIME_RANGE_SECONDS,
     BanOrigin,
     BansByCountryResponse,
+    BansByJailResponse,
     BanTrendBucket,
     BanTrendResponse,
     DashboardBanItem,
     DashboardBanListResponse,
+    JailBanCount,
     TimeRange,
     _derive_origin,
     bucket_count,
@@ -573,3 +575,70 @@ async def ban_trend(
         buckets=buckets,
         bucket_size=BUCKET_SIZE_LABEL[range_],
     )
+
+
+# ---------------------------------------------------------------------------
+# bans_by_jail
+# ---------------------------------------------------------------------------
+
+
+async def bans_by_jail(
+    socket_path: str,
+    range_: TimeRange,
+    *,
+    origin: BanOrigin | None = None,
+) -> BansByJailResponse:
+    """Return ban counts aggregated per jail for the selected time window.
+
+    Queries the fail2ban database ``bans`` table, groups records by jail
+    name, and returns them ordered by count descending.  The origin filter
+    is applied when provided so callers can restrict results to blocklist-
+    imported bans or organic fail2ban bans.
+
+    Args:
+        socket_path: Path to the fail2ban Unix domain socket.
+        range_: Time-range preset (``"24h"``, ``"7d"``, ``"30d"``, or
+            ``"365d"``).
+        origin: Optional origin filter — ``"blocklist"`` restricts to the
+            ``blocklist-import`` jail, ``"selfblock"`` excludes it.
+
+    Returns:
+        :class:`~app.models.ban.BansByJailResponse` with per-jail counts
+        sorted descending and the total ban count.
+    """
+    since: int = _since_unix(range_)
+    origin_clause, origin_params = _origin_sql_filter(origin)
+
+    db_path: str = await _get_fail2ban_db_path(socket_path)
+    log.info(
+        "ban_service_bans_by_jail",
+        db_path=db_path,
+        since=since,
+        range=range_,
+        origin=origin,
+    )
+
+    async with aiosqlite.connect(f"file:{db_path}?mode=ro", uri=True) as f2b_db:
+        f2b_db.row_factory = aiosqlite.Row
+
+        async with f2b_db.execute(
+            "SELECT COUNT(*) FROM bans WHERE timeofban >= ?" + origin_clause,
+            (since, *origin_params),
+        ) as cur:
+            count_row = await cur.fetchone()
+            total: int = int(count_row[0]) if count_row else 0
+
+        async with f2b_db.execute(
+            "SELECT jail, COUNT(*) AS cnt "
+            "FROM bans "
+            "WHERE timeofban >= ?"
+            + origin_clause
+            + " GROUP BY jail ORDER BY cnt DESC",
+            (since, *origin_params),
+        ) as cur:
+            rows = await cur.fetchall()
+
+    jails: list[JailBanCount] = [
+        JailBanCount(jail=str(row["jail"]), count=int(row["cnt"])) for row in rows
+    ]
+    return BansByJailResponse(jails=jails, total=total)