Add -a <instance> flag to all Authentik wrapper scripts, matching the
existing multi-instance pattern used by Woodpecker and Cloudflare.
credentials.json now supports per-instance Authentik config:
authentik.<instance>.url — instance URL
authentik.<instance>.token — API token (admin wrappers)
authentik.<instance>.test_user — username/password (Playwright/agent tests)
authentik.default — default instance name
Legacy flat structure (authentik.url) still works as fallback.
Token cache is now per-instance (~/.cache/mosaic/authentik-token-<name>).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
