Each internal-model functionality can now point at a rotation or autoselect
instead of the local model, falling back to the local model when the override is
empty or fails. Per-functionality fields in internal_model (default empty):
condensation_override, autoselect_override, nsfw_classifier_override,
privacy_classifier_override.

- handlers.py: run_meta_target() runs a global rotation/autoselect and returns
  the assistant text (or None -> fallback); classification overrides prompt the
  chat model for a strict YES/NO and parse it, falling back to the local
  classifier; autoselect selection can run through an override.
- context.py: condensation prefers the override, falling back to the local model.
- settings page: per-field override inputs backed by a datalist of available
  rotations/autoselects; saved to internal_model.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>