Add accessor key authorization and enhance completion logic

Introduce `backtunnel-authorize` for managing restricted SFTP-only keys, and update `backtunnel-share` to support temporary accessor key authorization via `--allow-key` and `--allow-known`. Extend bash completion with profile, accessor, and SSH host suggestions. Revamp README sections to include updated workflows, quick starts, and key management details.

scripts/backtunnel-authorize

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+name="${1:-}"
+file="${2:-}"
+[[ -n "$name" && -n "$file" && -f "$file" ]] || { echo "Usage: backtunnel-authorize <name> <pubkey-file>"; exit 1; }
+dir="${XDG_CONFIG_HOME:-$HOME/.config}/backtunnel/authorized"
+mkdir -p "$dir"
+install -m 644 "$file" "$dir/$name.pub"
+echo "Saved: $dir/$name.pub"