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/migrations/0005_uploaded_input.down.sql

@@ -0,0 +1,9 @@
+BEGIN;
+
+ALTER TABLE tasks DROP CONSTRAINT IF EXISTS ck_tasks_has_input;
+
+-- Restoring NOT NULL requires the columns to be populated; safe on a fresh DB.
+ALTER TABLE tasks ALTER COLUMN input_uri SET NOT NULL;
+ALTER TABLE jobs  ALTER COLUMN input_uri SET NOT NULL;
+
+COMMIT;

coordinator/migrations/0005_uploaded_input.up.sql

@@ -0,0 +1,13 @@
+BEGIN;
+
+-- Inputs can now arrive as uploaded artifacts (POST /jobs/upload), not only as
+-- external URIs. Relax the URI requirement and require every task to have an
+-- input one way or the other.
+ALTER TABLE jobs  ALTER COLUMN input_uri DROP NOT NULL;
+ALTER TABLE tasks ALTER COLUMN input_uri DROP NOT NULL;
+
+ALTER TABLE tasks ADD CONSTRAINT ck_tasks_has_input CHECK (
+    input_uri IS NOT NULL OR input_artifact_id IS NOT NULL
+);
+
+COMMIT;