Make Synapse's `enable_local_media_storage` configurable

Ref:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4882
- https://github.com/element-hq/synapse/pull/19204
- https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#enable_local_media_storage

We default it to `true`, keeping up with upstream and the old behavior.

s3-storage-provider users may set `matrix_synapse_enable_local_media_storage` to `false`
to disable local file caching.
This likely comes at the expense of some performance.

For matrix-media-repo users, it likely doesn't matter what this is set to,
as for a matrix-media-repo setup, all media-related API endpoints are
captured and forwarded to matrix-media-repo (before reaching Synapse).

roles/custom/matrix-synapse/defaults/main.yml

@@ -1092,6 +1092,11 @@ matrix_synapse_workers_media_repository_workers_container_arguments: []
 # Adjusting this value manually is generally not necessary.
 matrix_synapse_enable_media_repo: "{{ not matrix_synapse_ext_media_repo_enabled and (not matrix_synapse_workers_enabled or (matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'media_repository') | list | length == 0)) }}"
 
+# matrix_synapse_enable_local_media_storage controls whether the local on-disk media storage provider is enabled in Synapse.
+# When disabled, media is stored only in configured `media_storage_providers` and temporary files are used for processing (no local caching).
+# Warning: If this option is set to false and no `media_storage_providers` are configured, all media requests will return 404 errors as there will be no storage backend available.
+matrix_synapse_enable_local_media_storage: true
+
 # matrix_synapse_enable_authenticated_media controls if authenticated media is enabled.
 # If enabled all "old" media remains accessible over the legacy endpoints but new media is blocked.
 # while this option is enabled all media access and downloads have to be done via authenticated endpoints.

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1035,11 +1035,15 @@ federation_rr_transactions_per_room_per_second: {{ matrix_synapse_federation_rr_
 #enable_media_repo: false
 enable_media_repo: {{ matrix_synapse_enable_media_repo | to_json }}
 
+# Enable the local on-disk media storage provider.
+# When disabled, media is stored only in configured media_storage_providers and temporary files are used for processing (no local caching).
+# Warning: If this option is set to false and no media_storage_providers are configured, all media requests will return 404 errors as there will be no storage backend available.
+enable_local_media_storage: {{ matrix_synapse_enable_local_media_storage | to_json }}
+
 # Enable authenticated media.
 # enable_authenticated_media blocks access to new media from the legacy endpoints
 # and freezes the unauthenticated media repo by blocking all downloads that are not using
 # the new authenticated endpoints. If this option is turned off all media reverts to being considered "old"
-
 enable_authenticated_media: {{ matrix_synapse_enable_authenticated_media | to_json }}
 
 # Directory where uploaded images and attachments are stored.