Validate page param in Stats API breakdown endpoint

[nafey7]

Changes

The public Stats API GET /api/v1/stats/breakdown endpoint passed the page query parameter straight into String.to_integer/1, which raises ArgumentError on any non-numeric input — resulting in a 500 Internal Server Error.

This PR validates page the same way the sibling limit parameter is already validated (validate_or_default_limit/1): invalid or non-positive values now return a clean 400 with a descriptive message.

defp validate_or_default_page(%{"page" => page}) do
  case Integer.parse(page) do
    {page, ""} when page > 0 -> {:ok, page}
    _ -> {:error, "Please provide page as a number greater than 0."}
  end
end

defp validate_or_default_page(_), do: {:ok, 1}

Tests

Added tests to breakdown_test.exs covering non-integer, negative, and zero page values (mirroring the existing limit validation tests).

Notes

• Behavior for valid page values (including the default of 1 when omitted) is unchanged.
• CHANGELOG updated under a new Fixed section.

Fixes #6500

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