feat: add Stalwart Mail Server and FoundationDB library packages

- add stalwart: main mail server with systemd integration and user management
- add stalwart-cli: command line interface tool for server administration
- add stalwart-foundationdb: FoundationDB-enabled version with proper dependencies
- add libfoundationdb: FoundationDB client library (libfdb_c.so) package

All packages include proper conflict resolution, systemd services, and follow
repository packaging conventions. stalwart and stalwart-foundationdb are
mutually exclusive to prevent installation conflicts.

rpms/stalwart/resources/stalwart.service

@@ -0,0 +1,25 @@
+[Unit]
+Description=Stalwart Mail Server
+After=network.target
+
+[Service]
+Type=simple
+User=stalwart
+Group=stalwart
+ExecStart=/opt/stalwart/bin/stalwart --config=/opt/stalwart/etc/config.toml
+Restart=always
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=stalwart
+
+# Security settings
+NoNewPrivileges=true
+PrivateTmp=true
+PrivateDevices=true
+ProtectHome=true
+ProtectSystem=strict
+ReadWritePaths=/var/lib/stalwart /var/log/stalwart /opt/stalwart/data
+
+[Install]
+WantedBy=multi-user.target