feat: store backup config in DB secret instead of env var

Move backup SSH keys and repo URL from MCPD_BACKUP_REPO env var to a
"backup-ssh" secret in the database. Keys are auto-generated on first
init and stored back into the secret. Also fix ERR_HTTP_HEADERS_SENT
crash caused by reply.send() without return in routes when onSend hook
is registered.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

completions/mcpctl.bash

@@ -235,15 +235,12 @@ _mcpctl() {
     backup)
       local backup_sub=$(_mcpctl_get_subcmd $subcmd_pos)
       if [[ -z "$backup_sub" ]]; then
-        COMPREPLY=($(compgen -W "log key restore help" -- "$cur"))
+        COMPREPLY=($(compgen -W "log restore help" -- "$cur"))
       else
         case "$backup_sub" in
           log)
             COMPREPLY=($(compgen -W "-n --limit -h --help" -- "$cur"))
             ;;
-          key)
-            COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
-            ;;
           restore)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
             ;;