Stats API breakdown endpoint returns 500 on non-integer `page` parameter

[nafey7]

Description

The public Stats API GET /api/v1/stats/breakdown endpoint crashes with a 500 Internal Server Error when the page query parameter is not a valid positive integer.

The parameter is passed directly into String.to_integer/1:

https://github.com/plausible/analytics/blob/master/lib/plausible_web/controllers/api/external_stats_controller.ex#L44

page = String.to_integer(Map.get(params, "page", "1"))

String.to_integer/1 raises ArgumentError on any non-numeric input, so the request fails with a 500 instead of a helpful client error.

Steps to reproduce

GET /api/v1/stats/breakdown?site_id=<site>&property=event:page&page=foo

Also triggered by page=1.5, page= (empty), etc.

Expected behavior

A clean 400 Bad Request with a descriptive error message — consistent with the sibling limit parameter a few lines below, which is already validated via Integer.parse/1 (validate_or_default_limit/1).

Actual behavior

500 Internal Server Error (and a reported exception).

Proposed fix

Validate page the same way limit is validated, returning a 400 for invalid or non-positive values. Happy to open a PR.