Keep the recurring SCUM database poll bounded

Each dispatched poll is a durable job with two job log streams, so an
unbounded poll history would grow the platform store by roughly two
thousand jobs a day per server. A new poll now retires older terminal jobs
of the same template together with their job log streams, leaving at most
two rows per template while nothing is in flight.

platform/repo/resources.go

@@ -71,6 +71,7 @@ type JobRepository interface {
 	GetByIdempotency(runEndpointID string, idempotencyKey string) (domain.Job, error)
 	List(domain.JobFilter) ([]domain.Job, error)
 	Update(domain.Job) error
+	Delete(id string) error
 }
 
 type ArtifactRepository interface {
@@ -120,6 +121,7 @@ type LogStreamRepository interface {
 	Get(id string) (domain.LogStream, error)
 	List(domain.LogStreamFilter) ([]domain.LogStream, error)
 	Update(domain.LogStream) error
+	Delete(id string) error
 }
 
 type MetricSampleRepository interface {