Unable to create heatmaps while targeting internal/local host

Bug Description

In self-hosted version of Posthog there is a problem with heatmaps generation when target site is hosted on internal/private host.
I've found in source code that there is a call to block internal urls on heatmap creation process - https://github.com/PostHog/posthog/blob/300288a4e42036bb454355f6d4cc776683ee923e/posthog/tasks/heatmap_screenshot.py#L276
which may be necessary for cloud-based solution, but not in self-hosted.
The goal of self-hosted solutions is also to host them inside your own domain, maybe without internet access for security reasons.

How to reproduce

1. Connect self-hosted Posthog with site in local/internal domain
2. Open Posthog, navigate to 'Heatmaps'
3. Press 'New heatmap' button, then select site from step 1
4. Press 'Save'

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [x] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start in posthog/tasks/heatmap_screenshot.py around line 276 and trace how internal URLs are blocked during heatmap creation. Compare the behavior needed for self-hosted installations with the existing protection, then reproduce the reported flow using an internal target; done means self-hosted heatmap creation works for that target without breaking the relevant URL safeguards.