feat(rpm): create mistralrs system user and group on install

Add %pre scriptlet to ensure the mistralrs user and group exist
before the package files are installed. The systemd unit runs the
service as this user.

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

rpm/mistralrs.spec

@@ -58,6 +58,10 @@ sed -i "s|@BINARY@|%{_bindir}/mistralrs-server|g" \
 sed -i "s|@FLAVOUR@|%{mistralrs_flavour}|g" \
     %{buildroot}%{_unitdir}/mistralrs@.service
 
+%pre
+getent group mistralrs >/dev/null || groupadd -r mistralrs
+getent passwd mistralrs >/dev/null || useradd -r -g mistralrs -d /var/lib/mistralrs -s /sbin/nologin mistralrs
+
 %post
 %systemd_post mistralrs@.service