Synapse only generates thumbnails of local media at upload time (unless
dynamic_thumbnails is enabled, which the playbook does not do) and no
tooling exists to regenerate them, so restoring a backup left all
previously uploaded local images without thumbnails permanently. The
official Synapse backup guide recommends backing this directory up.

Related to #4004.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>