runpod: capability pods default to the published images

Now that ghcr.io/nextime/coderai-<capability> is public, a capability pod
needs no image name: leaving it blank uses the published image for that
capability. Enabling a capability on RunPod is one dropdown.

Capabilities that ride another profile's dependency set are mapped —
rerank to embeddings, speaker to stt, stems/audio_clean/audio_gen to
audio. The four with no published image (loras, characters, environments,
pipelines) return no default and say so, rather than defaulting to a tag
that 404s minutes into a pod boot.

CODERAI_CAPABILITY_IMAGE_REPO / _TAG point the whole set at a fork or a
private registry; a row's own `image` still wins. The UI shows the
default as each row's placeholder, so it is visible rather than implied.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY