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/internal/usecase/dto.go

@@ -49,6 +49,21 @@ type CompleteTaskInput struct {
 	Metrics          map[string]any
 }
 
+type SubmitDatasetInput struct {
+	Workload     string
+	Parameters   map[string]any
+	RowsPerShard int
+	Filename     string
+	ContentType  string
+	Body         io.Reader
+}
+
+type SubmitDatasetResult struct {
+	JobID           uuid.UUID
+	TaskCount       int
+	InputArtifactID uuid.UUID
+}
+
 type UploadArtifactInput struct {
 	TaskID      uuid.UUID
 	WorkerID    string