Extract SSH key import out of base.sh into a standalone, re-runnable service

Was only ever runnable once, buried inside base.sh's required-setup flow —
no way to re-run just this step for a box that already went through base
setup but needs another admin's key added later, or (the immediate case)
a home box for services/vpn-data-mount.sh that only needs this one step.

services/ssh-key-import.sh holds the real logic now (GitHub/Launchpad
import via ssh-import-id, optional password-auth lockdown); base.sh's
_base_setup_ssh chains into it the same way services/asterisk.sh chains
into security-dashboard/pstn-trunk, with a degraded (no import, just
ensures the SSH server itself is running) fallback for a pure standalone
`sudo bash base.sh` run with no sibling files sourced. Independently
runnable via `sudo ./setup.sh ssh-key-import` or `sudo bash
services/ssh-key-import.sh`, and shows up in the whiptail menu under
extras alongside ssh-config. Marked as never showing [installed] in
is_installed()/install_count(), same as ssh-config — it's a repeatable
management action, not a thing with an install state.

setup.sh

@@ -101,6 +101,7 @@ is_installed() {
         pstn-trunk) [ -f "$DOCKER_DIR/asterisk-digital-ocean/config/asterisk/pstn-trunk-pjsip.conf" ] || [ -f "$DOCKER_DIR/asterisk/config/asterisk/pstn-trunk-pjsip.conf" ] ;;
         sms-inbound) [ -f /opt/sms-inbound/settings.env ] ;;
         ssh-config) false ;;   # repeatable management tool, never shows [installed]
+        ssh-key-import) false ;;   # repeatable management tool, never shows [installed]
         # Every WordPress site is named from the first one on (no plain
         # $DOCKER_DIR/wordpress dir the default case below could match) —
         # [installed] means "at least one site exists", not any specific one.
@@ -122,7 +123,7 @@ is_installed() {
 # is_installed() as 0 or 1.
 install_count() {
     case "$1" in
-        base|glow|crowdsec|security-dashboard|kdeconnect|silent-send|sync-cc|sky-cam|sky-cam-frigate|asterisk|pstn-trunk|sms-inbound|ssh-config)
+        base|glow|crowdsec|security-dashboard|kdeconnect|silent-send|sync-cc|sky-cam|sky-cam-frigate|asterisk|pstn-trunk|sms-inbound|ssh-config|ssh-key-import)
             is_installed "$1" && echo 1 || echo 0 ;;
         wordpress)
             find "$DOCKER_DIR" -mindepth 1 -maxdepth 1 -name 'wordpress-*' -type d 2>/dev/null | wc -l ;;