Warn at startup when worker or cascade limits are far past the cores

More than four times os.cpu_count() logs one warning; the value is kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/tests/flow/test_resources.py

@@ -20,6 +20,7 @@ from fluksio.flow.resources import (
     fair_share_env,
     machine_gpus,
     machine_ram_mb,
+    warn_oversubscribed,
 )
 from fluksio.flow.schemas import NodeDef, Resources
 
@@ -45,6 +46,16 @@ def test_the_cards_are_counted_from_the_devices_the_driver_makes(monkeypatch):
     assert machine_gpus() == 0
 
 
+def test_a_limit_far_past_the_cores_is_warned_about(monkeypatch, caplog):
+    """Said once, never clamped: nodes waiting on devices can want a high one."""
+    monkeypatch.setattr(os, "cpu_count", lambda: 2)
+    warn_oversubscribed({"FLOW_MAX_WORKERS": 8, "FLOW_MAX_CASCADES": 9})
+
+    assert [r.getMessage() for r in caplog.records] == [
+        "Concurrency above 4x this machine's 2 cores: FLOW_MAX_CASCADES=9"
+    ]
+
+
 def test_memory_is_the_containers_limit_where_that_is_lower(monkeypatch, tmp_path):
     """The host's total is memory a container would be killed for using."""
     from fluksio.flow import resources