feat(coordinator): running state, worker liveness, request-size limits

Polish pass hardening the queue and closing plan gaps.

- Task state machine gains `running`: the first heartbeat moves a task from
  leased to running (migrations 0006/0007 add the enum value and extend the
  lease-integrity check). verifyLease, ExpireLease, the reaper SQL, and job
  progress all treat leased and running alike.
- Worker liveness: a heartbeat from a registered worker (UUID worker_id) bumps
  its last_heartbeat_at online; a second background reaper marks workers offline
  after WORKER_OFFLINE_AFTER of silence (RunReaper generalized to RunPeriodic).
- Request-size limits: JSON bodies capped at 1 MiB; dataset/artifact uploads
  capped at MAX_UPLOAD_BYTES (default 1 GiB) via http.MaxBytesReader.
- Tests cover the running transition, liveness + offline reaper (unit over
  memstore and integration over Postgres).

coordinator/internal/usecase/task.go

@@ -69,13 +69,15 @@ func (uc *ClaimTask) Execute(ctx context.Context, in ClaimTaskInput) (*domain.Cl
 
 type RenewLease struct {
 	tasks         TaskRepository
+	workers       WorkerRepository
 	tx            TxManager
 	clock         Clock
 	leaseDuration time.Duration
 }
 
-func NewRenewLease(tasks TaskRepository, tx TxManager, clock Clock, leaseDuration time.Duration) *RenewLease {
-	return &RenewLease{tasks: tasks, tx: tx, clock: clock, leaseDuration: leaseDuration}
+func NewRenewLease(tasks TaskRepository, workers WorkerRepository, tx TxManager,
+	clock Clock, leaseDuration time.Duration) *RenewLease {
+	return &RenewLease{tasks: tasks, workers: workers, tx: tx, clock: clock, leaseDuration: leaseDuration}
 }
 
 // Execute is a read-modify-write, so it runs inside a transaction with the row
@@ -101,6 +103,12 @@ func (uc *RenewLease) Execute(ctx context.Context, in RenewLeaseInput) (*domain.
 	if err != nil {
 		return nil, err
 	}
+
+	// Best-effort worker liveness, outside the task transaction so it can never
+	// fail the heartbeat. Only registered workers (a UUID worker_id) are tracked.
+	if id, perr := uuid.Parse(in.WorkerID); perr == nil {
+		_ = uc.workers.Touch(ctx, id, uc.clock.Now())
+	}
 	return &claimed, nil
 }