Add POST /runs/import for runs executed elsewhere

A history recorded before the move (MLflow experiments) had nowhere to
go, since runs were only made by running a flow. Each entry is stored as
it ended: parameters completed and checked like a submit, cause
"import", empty commit and code digest, metrics and artifacts written
as given, nothing queued or announced. Run.external_id (new migration)
makes a second import a no-op and is an export column so a row joins
back. The request is all or nothing and capped at 200 entries;
Client.import_runs pages a longer history.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QgG6TJw5qBZzaLghQSRVrq

docs/code/api.md

@@ -103,6 +103,7 @@ published to. Flows own the namespace; everything else is a client of it.
 |---|---|---|
 | `POST` | `/runs/flows/{name}` | queue one run — `{"params": {...}, "seed": 7, "draft": false, "no_cache": false}`. `"cause"` says where it came from — `api` (the default), `cli` or `sdk` |
 | `POST` | `/runs/flows/{name}/sweep` | queue many, sharing a `group_id` |
+| `POST` | `/runs/import` | record runs executed somewhere else, up to 200 per request. Answers `[{"external_id", "id", "created"}]` |
 | `GET` | `/runs` | the queryable history: `?flow=`, `?status=`, `?group=`, `?digest=`, `?since=`, `?before=`, `?limit=`, `?offset=` |
 | `DELETE` | `/runs?flow=` | every run of one flow, and everything hanging off them. `flow` is required. Refused with 409 while one of them is still going |
 | `GET` | `/runs/overview` | one row per flow that has runs, with how many are running or queued |
@@ -120,6 +121,16 @@ Submitting answers immediately with a `queued` run. Wrong parameters (an
 undeclared name, a value of the wrong type) come back as a 422 naming the
 problem, before anything executes.
 
+`/runs/import` takes `{"runs": [...]}`. Each entry carries `flow`,
+`external_id`, `params`, `seed`, `status` (`ok`, `error` or `cancelled`),
+`created_at`, `started_at`, `finished_at`, `result`, `metrics` (`name`,
+`step`, `ts`, `value`), `artifacts` (`name`, `filename`, `media_type`,
+`content_base64`), `labels` and `group_id`. The parameters are completed and
+checked as a submit's are, the run is stored with the cause `import` and an
+empty commit and code digest, and nothing is queued. The request is checked
+whole before anything is written, and an `external_id` already imported
+answers with that run and `"created": false`.
+
 `?digest=` filters by the hash of the parameters, which is how you find "every
 run that used exactly this configuration".