Adds the missing write path for the per-task model_override column (which
was previously only settable via manual SQL) and pairs it with a
provider_override so cross-provider switches resolve correctly:
- kanban_db: provider_override column (+migration), set_model_override()
with model_override_set event, create_task(model_override=,
provider_override=), dispatcher spawns worker with -m <model>
[--provider <name>]
- dashboard: Model row in the task drawer — dropdown fed by a new
/model-options endpoint (build_models_payload substrate, provider-grouped,
free-text fallback), PATCH + bulk model override support
- CLI: kanban create --model/--provider, new kanban set-model subcommand,
show prints the provider
- agent tools: kanban_create accepts model/provider; show/list expose
provider_override
Rate-limit recovery flow: override is settable on running tasks and takes
effect on the next dispatch, without touching the worker profile's config.
