(feature) Add host doctor checks and Django log viewer

Add host-level checks for address, enabled state, SSH credential
selection, and backup directory readiness, and show them on the host
detail page.

Create host backup directories during host creation and prefill new
hosts from the default global config.

Add a staff-only logs view backed by journalctl with filtering by
pobsync unit, priority, and message text.

Improve runtime checks for gunicorn in virtualenv installs and ensure
the native installer grants the service user access to the backup root.

scripts/install-systemd

@@ -344,7 +344,7 @@ else
 fi
 
 run_step "Prepare directories" mkdir -p /etc/pobsync /var/lib/pobsync /var/log/pobsync "$(dirname "$VENV_DIR")" "$APP_DIR" "$BACKUP_ROOT"
-run_step "Set state directory permissions" chown "$SERVICE_USER:$SERVICE_GROUP" /var/lib/pobsync /var/log/pobsync
+run_step "Set state directory permissions" chown "$SERVICE_USER:$SERVICE_GROUP" /var/lib/pobsync /var/log/pobsync "$BACKUP_ROOT"
 run_step "Set private directory modes" chmod 0750 /var/lib/pobsync /var/log/pobsync
 
 if [ "$SOURCE_DIR" != "$APP_DIR" ]; then