Add support for CyberTipLine/PhotoDNA (Image fingerprinting for CSAM detection and reporting)

*This issue was originally created by [**@paigeadelethompson**](https://github.com/paigeadelethompson) at .*

Support for this should really be added to home servers. This is actually something that CloudFlare supports although Cloud Flare is unlikely to fix the problem. There needs to be something for home servers to prevent their users from uploading abhorrent material even if it's just best effort.

https://www.microsoft.com/en-us/photodna
https://blog.cloudflare.com/the-csam-scanning-tool/
https://report.cybertip.org/ws-hashsharing/v2/documentation/#overview
https://report.cybertip.org/ispws/documentation/#curl-example

The issue names no files, tests, or entry points. Start by reviewing the PhotoDNA and CyberTipLine documentation linked in the issue, then locate the homeserver media-upload flow and determine the reporting requirements. Done means a documented, best-effort path for fingerprinting and CSAM reporting, with its scope and limitations agreed by maintainers.

Written by the indexing model from the issue text.