Add mass unban: DELETE /api/bans/all clears all active bans

- Send fail2ban's `unban --all` command via new `unban_all_ips()` service
  function; returns the count of unbanned IPs
- Add `UnbanAllResponse` Pydantic model (message + count)
- Add `DELETE /api/bans/all` router endpoint; handles 502 on socket error
- Frontend: `bansAll` endpoint constant, `unbanAllBans()` API call,
  `UnbanAllResponse` type, `unbanAll` action in `useActiveBans` hook
- JailsPage: "Clear All Bans" button (visible when bans > 0) with a
  Fluent UI confirmation Dialog before executing the operation
- 7 new tests (3 service, 4 router); 440 total pass, 82% coverage

backend/tests/test_routers/test_bans.py

@@ -13,6 +13,7 @@ from app.config import Settings
 from app.db import init_db
 from app.main import create_app
 from app.models.ban import ActiveBan, ActiveBanListResponse
+from app.utils.fail2ban_client import Fail2BanConnectionError
 
 # ---------------------------------------------------------------------------
 # Fixtures
@@ -270,3 +271,61 @@ class TestUnbanIp:
             )
 
         assert resp.status_code == 404
+
+
+# ---------------------------------------------------------------------------
+# DELETE /api/bans/all
+# ---------------------------------------------------------------------------
+
+
+class TestUnbanAll:
+    """Tests for ``DELETE /api/bans/all``."""
+
+    async def test_200_clears_all_bans(self, bans_client: AsyncClient) -> None:
+        """DELETE /api/bans/all returns 200 with count when successful."""
+        with patch(
+            "app.routers.bans.jail_service.unban_all_ips",
+            AsyncMock(return_value=3),
+        ):
+            resp = await bans_client.request("DELETE", "/api/bans/all")
+
+        assert resp.status_code == 200
+        data = resp.json()
+        assert data["count"] == 3
+        assert "3" in data["message"]
+
+    async def test_200_with_zero_count(self, bans_client: AsyncClient) -> None:
+        """DELETE /api/bans/all returns 200 with count=0 when no bans existed."""
+        with patch(
+            "app.routers.bans.jail_service.unban_all_ips",
+            AsyncMock(return_value=0),
+        ):
+            resp = await bans_client.request("DELETE", "/api/bans/all")
+
+        assert resp.status_code == 200
+        assert resp.json()["count"] == 0
+
+    async def test_502_when_fail2ban_unreachable(
+        self, bans_client: AsyncClient
+    ) -> None:
+        """DELETE /api/bans/all returns 502 when fail2ban is unreachable."""
+        with patch(
+            "app.routers.bans.jail_service.unban_all_ips",
+            AsyncMock(
+                side_effect=Fail2BanConnectionError(
+                    "cannot connect",
+                    "/var/run/fail2ban/fail2ban.sock",
+                )
+            ),
+        ):
+            resp = await bans_client.request("DELETE", "/api/bans/all")
+
+        assert resp.status_code == 502
+
+    async def test_401_when_unauthenticated(self, bans_client: AsyncClient) -> None:
+        """DELETE /api/bans/all returns 401 without session."""
+        resp = await AsyncClient(
+            transport=ASGITransport(app=bans_client._transport.app),  # type: ignore[attr-defined]
+            base_url="http://test",
+        ).request("DELETE", "/api/bans/all")
+        assert resp.status_code == 401