fix(rpm): use sysusers.d for cortex user/group creation

Both packages set %attr(...,cortex) on their config files, which
caused RPM's auto-dep-generator to emit Requires: group(cortex) /
user(cortex). The %pre scriptlets that actually created the group
ran too late — dnf rejected neuron installation on hosts without
cortex because nothing Provided group(cortex).

Switch to systemd-sysusers declarative user creation: each package
ships its own named sysusers.d file (cortex-gateway.conf and
cortex-neuron.conf — different names so both packages can coinstall)
with identical content defining the cortex user/group. RPM's
user/group dep generator now emits Provides: user(cortex) and
Provides: group(cortex) automatically from the sysusers.d files,
satisfying the auto-generated Requires. Either package installs
standalone; both can coinstall on the gateway host if desired.

Also added Requires: systemd since %sysusers_create_compat depends
on systemd-sysusers being present on the target.

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

data/cortex-sysusers.conf

@@ -0,0 +1,3 @@
+g cortex - -
+u cortex - "Cortex inference cluster" /var/lib/cortex /sbin/nologin
+m cortex cortex