runpod: optional network volumes for weights, uploads and staging

A pod's disk dies with the pod, so every cold pod re-downloads its
weights and every upload has to be repeated. A network volume — created
by hand in the console, coderai never creates one — makes that a one-off.

Attaching it is the easy half; the useful half is pointing HF_HOME, the
diffusers/transformers caches, CODERAI_MODELS_DIR and the staging
directory at the mount. A volume attached without redirecting those
changes nothing, which is the mistake this avoids.

That also answers the bounce-host question: an upload lands on the volume
and every later pod sees it, so a QLoRA or a local model can be sent once
and used from any pod — no third-party storage, no public IP.

Two RunPod constraints are enforced rather than discovered the hard way:
volumes are Secure Cloud only (cloud_types is forced), and a pod must run
in the volume's own data center (looked up once and pinned, instead of
letting price ranking pick a region where the attach fails as a confusing
capacity error).

Settable account-wide in Settings or per pool. Container images still
come from a registry — a volume holds weights, not the image.

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