b405c08fcc
Set `id` as key for the association between `InferenceRouter` and `Strand`.
Without explicitly setting the key, Sequel assumes that `Strand` has a
column `inference_router_id`, which is not the case:
```
> InferenceRouter.first.strand
Sequel::DatabaseError: PG::UndefinedColumn: ERROR: column strand.inference_router_id does not exist
LINE 1: SELECT * FROM "strand" WHERE ("strand"."inference_router_id"...
```