Stop excluding Synapse's local_thumbnails from BorgBackup

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>

CHANGELOG.md

@@ -1,3 +1,13 @@
+# 2026-07-14
+
+## BorgBackup now includes Synapse's local thumbnails
+
+For Synapse servers, the built-in [BorgBackup](./docs/configuring-playbook-backup-borg.md) integration no longer excludes the media store's `local_thumbnails` directory from backups.
+
+Synapse only generates thumbnails of local media at upload time (unless `dynamic_thumbnails` is enabled, which the playbook does not do), and there is no tooling to regenerate them. Restoring a backup made with the previous exclusion list therefore left all previously uploaded local images without thumbnails. The [official Synapse backup guide](https://element-hq.github.io/synapse/latest/usage/administration/backups.html) recommends backing this directory up, and the playbook now follows that recommendation.
+
+Expect your backups to grow somewhat, depending on how much image media your local users have uploaded. If you prefer the old behavior, you can redefine `backup_borg_location_exclude_patterns` in your `vars.yml`.
+
 # 2026-07-12
 
 ## matrix-registration-bot has been removed from the playbook

group_vars/matrix_servers

@@ -3378,9 +3378,11 @@ backup_borg_postgresql_databases_auto: "{{ postgres_managed_databases | map(attr
 backup_borg_location_source_directories:
   - "{{ matrix_base_data_path }}"
 
+# local_thumbnails is deliberately NOT excluded: with dynamic_thumbnails disabled (Synapse's default),
+# nothing regenerates lost thumbnails, so the official Synapse backup guide recommends backing them up.
 backup_borg_location_exclude_patterns: |
   {{
-    ([matrix_synapse_media_store_path + '/remote_content', matrix_synapse_media_store_path + '/local_thumbnails', matrix_synapse_media_store_path + '/remote_thumbnail', matrix_synapse_media_store_path + '/url_cache', matrix_synapse_media_store_path + '/url_cache_thumbnails'] if matrix_homeserver_implementation == 'synapse' else [])
+    ([matrix_synapse_media_store_path + '/remote_content', matrix_synapse_media_store_path + '/remote_thumbnail', matrix_synapse_media_store_path + '/url_cache', matrix_synapse_media_store_path + '/url_cache_thumbnails'] if matrix_homeserver_implementation == 'synapse' else [])
     +
     ([postgres_data_path] if postgres_enabled else [])
   }}