The model-list discovery flow (POST create → poll GET) used an in-memory
map, so in multi-instance deployments the poll could land on a different
pod and return 404. Move the store to a database table so all pods share
the same state.
Changes:
- Add migration 068: model_list_request table with partial index
- Add sqlc queries with FOR UPDATE SKIP LOCKED for atomic PopPending
- Introduce ModelListStorer interface; DBModelListStore for production,
InMemoryModelListStore for unit tests
- Update model-dropdown and model-picker to show "discovery failed"
instead of misleading "No models available" on error
Co-authored-by: multica-agent <github@multica.ai>
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