feat: switch from deprecated mistralrs-server to mistralrs CLI

Build the mistralrs binary (CLI) instead of the deprecated
mistralrs-server. The RPM still installs as /usr/bin/mistralrs-server
for backwards compatibility. The systemd unit now invokes
`mistralrs-server serve` to use the CLI's serve subcommand.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

rpm/systemd/mistralrs@.service

@@ -9,7 +9,7 @@ User=mistralrs
 Group=mistralrs
 SupplementaryGroups=video render
 EnvironmentFile=/etc/mistralrs/%i.conf
-ExecStart=@BINARY@ $MISTRALRS_ARGS
+ExecStart=@BINARY@ serve $MISTRALRS_ARGS
 Restart=on-failure
 RestartSec=10s