feat(coordinator): upload a dataset and chunk it into shard tasks (CTX-05, part 4)

The coordinator can now ingest a dataset itself, not only accept client-supplied
chunk URIs.

- internal/chunk: a deterministic, generic TSV row splitter — repeats the header
  per shard, buffers one shard at a time, rejects header-only input. Unit-tested.
- POST /jobs/upload (multipart): streams the dataset into an input artifact,
  splits it into shard artifacts, and creates one shard task per shard, all in
  one transaction; blobs are cleaned up if the transaction fails.
- GET /tasks/{id}/input streams a task's input shard back to the worker.
- domain: NewUploadedJob, NewShardTask, Task/Job.InputArtifactID; a shard task's
  input is an artifact, not a URI. Claim response nests input:{uri,sha256} per
  the contract, with uri = /tasks/{id}/input for shards.
- migration 0005 makes input_uri nullable and adds a has-input check.
- The existing URI-based POST /jobs path is untouched; both coexist.

coordinator/cmd/coordinator/main.go

@@ -70,6 +70,7 @@ func run() error {
 	useCases := httptransport.UseCases{
 		RegisterWorker:   usecase.NewRegisterWorker(workerRepo, clk),
 		CreateJob:        usecase.NewCreateJob(jobRepo, taskRepo, tx, clk),
+		SubmitDataset:    usecase.NewSubmitDataset(blobStore, artifactRepo, jobRepo, taskRepo, tx, clk),
 		ClaimTask:        usecase.NewClaimTask(taskRepo, clk, cfg.LeaseDuration),
 		RenewLease:       usecase.NewRenewLease(taskRepo, tx, clk, cfg.LeaseDuration),
 		CompleteTask:     usecase.NewCompleteTask(taskRepo, jobRepo, artifactRepo, tx, clk),
@@ -77,6 +78,7 @@ func run() error {
 		GetJobStatus:     usecase.NewGetJobStatus(jobRepo, taskRepo),
 		UploadArtifact:   usecase.NewUploadArtifact(taskRepo, artifactRepo, blobStore, clk),
 		DownloadArtifact: usecase.NewDownloadArtifact(artifactRepo, blobStore),
+		GetTaskInput:     usecase.NewGetTaskInput(taskRepo, artifactRepo, blobStore),
 	}
 
 	// Background workers are tracked so shutdown can wait for them. Without