Run child tables cascade from run.id

Migration ccb21eacf5d5 deletes rows whose run is gone, then adds foreign keys
with ON DELETE CASCADE from run_node, run_metric and run_artifact to run.id.
Deleting runs is now the one statement. A node or metric written after its
run was deleted is dropped with an info line. Migrations run with foreign keys
off, since a batch rebuild of run would otherwise cascade into every child row.

backend/tests/api/routes/test_runs.py

@@ -7,6 +7,7 @@ The pipeline half — what a hit restores and what a key is made of — is in
 import base64
 import csv
 import json
+import logging
 import threading
 from datetime import UTC, datetime, timedelta
 
@@ -41,6 +42,10 @@ def test_a_run_cache_finds_what_an_earlier_run_recorded(tmp_path):
     reference = store.put([b"payload"], name="data.bin")
     plain, with_artifact, collected = "k-plain", "k-artifact", "k-collected"
     with Session(db_engine) as session:
+        for run_id in ("cache-1", "cache-2", "cache-3"):
+            session.add(
+                Run(id=run_id, flow="study", status="ok", created_at=datetime.now(UTC))
+            )
         session.add(
             RunNode(
                 run_id="cache-1",
@@ -891,7 +896,7 @@ def test_overview_is_not_read_as_a_run_id(client, superuser_token_headers):
 # -----------------------------------------------------------------------------
 # Deleting a run
 #
-# The route owns the four statements; what these guard is that it takes the
+# The foreign keys cascade; what these guard is that deleting a run takes the
 # children with it and refuses a run the driver is still writing to.
 # -----------------------------------------------------------------------------
 
@@ -928,7 +933,7 @@ def deletable_run():
 def test_deleting_a_run_takes_its_children_with_it(
     client, superuser_token_headers, deletable_run
 ):
-    """No foreign key cascades here, so the route has to do it itself."""
+    """The foreign keys cascade, which needs them on for every connection."""
     answer = client.delete(
         f"{settings.API_V1_STR}/runs/{deletable_run}", headers=superuser_token_headers
     )
@@ -943,6 +948,18 @@ def test_deleting_a_run_takes_its_children_with_it(
             assert left == [], f"{table.__name__} rows outlived the run"
 
 
+def test_a_node_finishing_after_its_run_was_deleted_is_dropped_quietly(caplog):
+    """The constraint refuses its row, and that is a no-op rather than an error."""
+    service = RunService(controller=_OneFlow(FlowDef(name="gone")), queue=_Collect())
+    with caplog.at_level(logging.INFO, logger="fluksio.flow.runs"):
+        service._record_node("del-gone", NodeOutcome(node="gone.a", ok=True))
+
+    with Session(db_engine) as session:
+        assert session.get(RunNode, {"run_id": "del-gone", "node": "gone.a"}) is None
+    assert "is gone" in caplog.text
+    assert not [record for record in caplog.records if record.levelno >= logging.ERROR]
+
+
 def test_deleting_a_run_that_is_not_there_is_a_404(client, superuser_token_headers):
     answer = client.delete(
         f"{settings.API_V1_STR}/runs/nope-1", headers=superuser_token_headers