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/concepts/runs.md

@@ -383,6 +383,39 @@ path (`metric.final_metrics.train_loss`) and `metrics="final_metrics.train_loss"
 selects one. Inputs work the same way, and are compared leaf by leaf, so two
 configurations differing in one field give that field as the axis.
 
+### Bringing in runs made elsewhere
+
+Runs are made by running a flow, so a history recorded before the move (an
+MLflow experiment, say) has to be imported instead. Each entry names the flow,
+the id the run had where it ran, and what it ended with:
+
+```python
+Client().import_runs([
+    {
+        "flow": "train",
+        "external_id": "mlflow:study-1/3f2a…",
+        "params": {"lr": 0.01},
+        "seed": 1001,
+        "status": "ok",
+        "created_at": "2026-03-01T10:00:00Z",
+        "started_at": "2026-03-01T10:00:01Z",
+        "finished_at": "2026-03-01T10:40:12Z",
+        "result": {"final_metrics": {"train_loss": 0.08}},
+        "metrics": [{"name": "train.train_loss", "step": 0, "ts": 1772359201.0, "value": 2.3}],
+        "artifacts": [{"name": "weights", "filename": "model.txt",
+                       "media_type": "text/plain", "content_base64": "…"}],
+        "labels": ["imported"],
+        "group_id": "mlflow:study-1",
+    },
+])
+```
+
+The parameters are completed with the flow's declared values and checked the
+way a submit checks them. The run is stored with the cause `import` and no
+commit or code digest, since this engine never saw the code it ran. Importing
+the same `external_id` again adds nothing, and `fluksio export runs` carries
+it as a column, so an exported row joins back to where it came from.
+
 ### A dashboard, read against runs
 
 A run records values under the same names a dashboard binds to (a run of