feat(coordinator): artifact upload/download endpoints (CTX-05, part 2)

Wire the artifact storage foundation to HTTP.

- PUT /tasks/{id}/artifacts/{filename}: a worker streams a partial result;
  the coordinator verifies lease ownership (foreign worker → 409), streams
  the bytes to blob storage while hashing, and records the metadata. An
  orphaned blob from a failed metadata insert is cleaned up.
- GET /artifacts/{id}/download: streams an artifact back with its content
  type, length, and checksum.
- Ownership is read with a new non-locking TaskRepository.Get, so no row lock
  is held across a long upload. Identity travels in X-Worker-ID / X-Task-Attempt
  headers per the contract; upload/download bypass the short request timeout.
- docker-compose mounts ./data for durable artifact storage; smoke and
  requests.http exercise an upload → foreign-409 → download round-trip.

coordinator/api/requests.http

@@ -80,6 +80,24 @@ Content-Type: application/json
   "attempt": {{attempt}}
 }
 
+### 3a. Upload a partial-result artifact (200) — while the task is leased
+# Identity travels in headers per the contract; the body is streamed as-is.
+# @name uploadArtifact
+PUT {{host}}/tasks/{{taskId}}/artifacts/result.csv
+Authorization: Bearer {{token}}
+Content-Type: text/csv
+X-Worker-ID: {{worker}}
+X-Task-Attempt: {{attempt}}
+
+query,match,score
+CHEMBL25,CHEMBL139,0.87
+
+@artifactId = {{uploadArtifact.response.body.artifact_id}}
+
+### 3b. Download the artifact by id (200)
+GET {{host}}/artifacts/{{artifactId}}/download
+Authorization: Bearer {{token}}
+
 ### 4. Submit the result (200)
 POST {{host}}/tasks/{{taskId}}/result
 Authorization: Bearer {{token}}