List models, per-role runtime defaults, and the system default fallback chain

Selectable models + the code-default model each runtime role falls back to.

The role defaults mirror the runtime constants (single-sourced via src.runtime.model_defaults.ROLE_DEFAULTS, which imports the SAME constants the runtime applies) so the UI can show what's actually used — model + effort + max_tokens + temperature + thinking budget — when an entity leaves a setting unset. Imported lazily inside the handler: model_router deliberately never imports src.runtime at module scope (the runtime imports model_router.service — a top-level import here risks a cycle).