Platform-agnostic refactor: eliminate OS-specific hardcodes from core scripts

All bash scripts are now platform-neutral. Unraid-specific paths, commands,
and service checks moved to Plugin/unraid/adapter.sh. Core scripts call
platform_*() functions exclusively — no direct OS paths in runtime logic.

New adapter functions: platform_storage_path, platform_webui_install_path,
platform_scripts_dir_probe_cmd, platform_setup_db_path, platform_storage_healthy,
platform_is_service_enabled, platform_get_temp_thresholds, platform_disk_states_path,
platform_rebuild_container, platform_push_conf, platform_push_setup_state,
platform_get_templates_dir, platform_send_os_notification.

Partnership services stack (Emby/Jellyfin/Seerr/SeerrFin) added as third
onboarding stack alongside auth and arr stacks.

Partnership/onboard_cancel.sh

@@ -88,7 +88,7 @@ if [[ "$DIRECTION" == "h1" || "$DIRECTION" == "both" ]]; then
             timeout "$SSH_TIMEOUT" ssh -i "$SSH_KEY" \
                 -o ConnectTimeout="$SSH_TIMEOUT" -o BatchMode=yes root@"$MIRROR_IP" \
                 "sed -i \"|${KEY_BLOB}|d\" /root/.ssh/authorized_keys 2>/dev/null
-                 sed -i \"/^${MIRROR_ID}_PHASE\|^${MIRROR_ID}_KEY_READY/d\" /boot/config/varaverk_setup.db 2>/dev/null
+                 sed -i \"/^${MIRROR_ID}_PHASE\|^${MIRROR_ID}_KEY_READY/d\" $(platform_setup_db_path) 2>/dev/null
                  echo ok" 2>/dev/null | grep -q ok && {
                 log "HOST1 key removed from $MIRROR authorized_keys ✅"
                 H1_DONE=true