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_services/test_jail_service.py

@@ -554,3 +554,38 @@ class TestLookupIp:
             result = await jail_service.lookup_ip(_SOCKET, "9.9.9.9")
 
         assert result["currently_banned_in"] == []
+
+
+# ---------------------------------------------------------------------------
+# unban_all_ips
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+class TestUnbanAllIps:
+    """Tests for :func:`~app.services.jail_service.unban_all_ips`."""
+
+    async def test_unban_all_ips_returns_count(self) -> None:
+        """unban_all_ips returns the integer count from fail2ban."""
+        responses = {"unban|--all": (0, 5)}
+        with _patch_client(responses):
+            count = await jail_service.unban_all_ips(_SOCKET)
+
+        assert count == 5
+
+    async def test_unban_all_ips_returns_zero_when_none_banned(self) -> None:
+        """unban_all_ips returns 0 when no IPs are currently banned."""
+        responses = {"unban|--all": (0, 0)}
+        with _patch_client(responses):
+            count = await jail_service.unban_all_ips(_SOCKET)
+
+        assert count == 0
+
+    async def test_unban_all_ips_raises_on_connection_error(self) -> None:
+        """unban_all_ips propagates Fail2BanConnectionError."""
+        with patch(
+            "app.services.jail_service.Fail2BanClient",
+            side_effect=Fail2BanConnectionError("unreachable", _SOCKET),
+        ):
+            with pytest.raises(Fail2BanConnectionError):
+                await jail_service.unban_all_ips(_SOCKET)