For a coderai instance that only orchestrates RunPod (no local models), the node
that hosts the proxy must advertise the runpod capability. The no-GPU path already
builds its engine with backend "auto" (which has it), but an explicit engine_specs
backend:"cpu" and the unknown-backend fallback did not — those would 503.

- _DEFAULT_CAPS gains "cpu": {transformers, gguf, k3, runpod}.
- Unknown-backend fallback now {transformers, gguf, runpod} (runpod needs no local
  hardware, so every node can host it).

Verified by simulating pick_engine for a runpod-pinned model on auto/cpu/nvidia
nodes — all route instead of 503.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R6iugoNeEgt9PkajKrFyq4