Add Synology DSM support (#5315)

Adds optional support for running the playbook on Synology DSM 7+, detected
automatically via /etc/synoinfo.conf so that non-Synology hosts are unaffected.

Includes DSM-native user/group management (synouser/synogroup), a requests
version constraint for Docker SDK compatibility, and a boot-fix service that
re-shares the volume mount and starts matrix services skipped by DSM's boot
ordering. The shared-mount volume path is configurable via
matrix_base_synology_volume_path, and the make-shared step only runs when the
volume is not already shared.

Co-authored-by: CKSit <sitchiuki@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

roles/custom/matrix-base/templates/systemd/matrix-synology-boot-fix.service.j2

@@ -0,0 +1,16 @@
+# SPDX-FileCopyrightText: 2026 Chiu Ki Sit
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+[Unit]
+Description=Matrix Services Boot Recovery (Synology)
+# Run after multi-user.target so all matrix services have been attempted first.
+After=multi-user.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart={{ matrix_bin_path }}/matrix-synology-boot-fix
+
+[Install]
+WantedBy=multi-user.target