Add mautrix-discord bridge config and installer

Bot-login Discord bridge on LXC 151: config template (DB password is
filled in on the server), systemd unit, and a one-time install script
run from the Proxmox node that sets up the PostgreSQL role/database,
installs the checksum-verified v0.7.7 binary, generates the appservice
registration, adds it to Synapse and restarts Synapse once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

systemd/mautrix-discord.service

@@ -0,0 +1,30 @@
+[Unit]
+Description=mautrix-discord Matrix-Discord bridge
+After=network-online.target matrix-synapse.service
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=mautrix-discord
+Group=mautrix-discord
+WorkingDirectory=/opt/mautrix-discord
+ExecStart=/opt/mautrix-discord/mautrix-discord -c /opt/mautrix-discord/config.yaml -r /opt/mautrix-discord/registration.yaml
+Restart=on-failure
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=mautrix-discord
+
+# Resource limits
+MemoryMax=512M
+CPUQuota=80%
+
+# Hardening
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+ReadWritePaths=/opt/mautrix-discord /var/log/mautrix-discord
+
+[Install]
+WantedBy=multi-user.target