Declare this machine's GPUs from serve, and refuse a bad limit as a flag

GPU count is not detected, so FLOW_GPUS was 0 on a fresh install and a node
asking for one was silently clamped to zero and ran concurrently with every
other. Setting the variable serialised them, but it was an environment
variable only — `serve` had --max-runs and --max-workers and no --gpus.
The clamp warning now names the flag when nothing here declares a card.

The same flags are written into the environment before the settings are
built, so a value they refused died in a pydantic import naming no flag.
They are checked where they are typed instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/fluksio/flow/placement.py

@@ -215,9 +215,18 @@ class Placer:
         said = [shape[2] for shape in capable if shape[2]]
         ram = min(ram, max(said)) if said and ram else ram
         if (cpus, gpus, ram) != (wanted.cpus, wanted.gpus, wanted.ram or 0):
+            # Cards are not detected, so a machine that has one still reports
+            # none until it is told — which reads as "no GPU here" to a node
+            # that then runs unserialised beside every other one.
+            hint = (
+                "; no machine here declares a GPU — `fluksio serve --gpus N` "
+                "(or FLOW_GPUS) says how many this one has"
+                if wanted.gpus and not gpus
+                else ""
+            )
             logger.warning(
                 "%s asked for %d cpu(s), %d gpu(s) and %s MB; "
-                "the largest machine here can give %d, %d and %s",
+                "the largest machine here can give %d, %d and %s%s",
                 node or "a node",
                 wanted.cpus,
                 wanted.gpus,
@@ -225,6 +234,7 @@ class Placer:
                 cpus,
                 gpus,
                 ram or "no stated",
+                hint,
             )
         return cpus, gpus, ram