fix(poller): the health panel must not blank the page, and pass the tunables through (v0.23.20-beta)

Two faults reported against v0.23.19, one of which makes the other much worse.

**POLLER_CONCURRENCY was never passed into the container.** It is read from the
environment by the backend and documented in .env.example, but docker-compose
did not forward it, so setting it did nothing at all. The same was true of the
job timeout, interval and both retention limits. Every poller tunable shipped in
v0.23.17 was inert for anyone running the compose stack — including the one knob
the #114 reply told the reporter to reach for. All five are now forwarded.

**The health panel crashed on an unexpected payload**, taking the whole page
with it: `cap is undefined` from reading data.capacity without a guard. It also
rendered "Loading…" indefinitely on a request error, so a failing endpoint and a
slow one looked identical.

Both are the wrong failure mode for this screen in particular. It is the page an
operator opens precisely when monitoring already looks broken, and a blank white
panel at that moment is worse than no panel: it suggests the whole tool is dead
rather than that one query failed.

Every field is now defaulted, so a backend a version behind or one that answered
partially degrades the panel instead of destroying the page, and a failed request
renders what actually went wrong with a retry. Verified against the four shapes
that could produce the reported crash: capacity missing, an error body arriving
with a 2xx, an older backend without the status field, and an empty object.

The endpoint is hardened to match. Device count, timing and the stale-device
listing are each guarded independently, so a partial failure returns partial
numbers rather than a 500 — the same reasoning applied at the other end.

I could not reproduce the original crash: at concurrency 48 the endpoint returns
a complete payload here, there is no route shadowing, and the axios interceptor
rejects errors correctly rather than passing them through as data. The defensive
work stands on its own regardless of which of those shapes he actually hit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/system/PollerHealthCard.tsx

@@ -50,12 +50,19 @@ function Stat({ label, value, hint, alarm }: {
   );
 }
 
+/** Fallback capacity, so a partial or unexpected payload cannot blank the page. */
+const EMPTY_CAPACITY: PollerHealth['capacity'] = {
+  devices: 0, avg_fast_poll_ms: null, p90_fast_poll_ms: null,
+  arrival_per_sec: 0, service_per_sec: null, headroom: null,
+  backlog: 0, drain_eta_sec: 0,
+};
+
 export default function PollerHealthCard() {
   const queryClient = useQueryClient();
   const [drainResult, setDrainResult] = useState('');
   const [confirmDrain, setConfirmDrain] = useState(false);
 
-  const { data, isLoading } = useQuery({
+  const { data, isLoading, isError, error } = useQuery({
     queryKey: ['poller-health'],
     queryFn: () => pollerApi.health().then(r => r.data),
     refetchInterval: 15_000,
@@ -70,14 +77,49 @@ export default function PollerHealthCard() {
     },
   });
 
-  if (isLoading || !data) {
+  if (isLoading) {
     return <div className="text-sm text-gray-500 dark:text-slate-400">Loading poller health…</div>;
   }
 
+  /**
+   * Failure has to be visible here of all places.
+   *
+   * This panel previously rendered "Loading…" forever on an error and threw
+   * outright on an unexpected payload, blanking the whole page — the worst
+   * possible behaviour for the one screen you open when monitoring looks broken.
+   * Whatever comes back, say something useful about it.
+   */
+  if (isError || !data || typeof data !== 'object') {
+    const msg = (error as { response?: { data?: { error?: string } }; message?: string } | null);
+    return (
+      <div className="rounded-lg border border-red-300 bg-red-50 p-4 text-sm dark:border-red-700/60 dark:bg-red-900/20">
+        <div className="flex items-center gap-2 font-medium text-red-800 dark:text-red-300">
+          <AlertTriangle className="h-4 w-4" /> Could not read poller health
+        </div>
+        <p className="mt-1 text-red-700 dark:text-red-300">
+          {msg?.response?.data?.error || msg?.message || 'The server returned an unexpected response.'}
+        </p>
+        <p className="mt-1 text-xs text-red-600 dark:text-red-400">
+          Polling itself may still be running — this panel only reports on it. Check the backend logs
+          if this persists.
+        </p>
+        <button onClick={() => queryClient.invalidateQueries({ queryKey: ['poller-health'] })}
+                className="btn-secondary mt-3 flex items-center gap-2 text-xs">
+          <RefreshCw className="h-3.5 w-3.5" /> Retry
+        </button>
+      </div>
+    );
+  }
+
+  // Every field below is defaulted. A backend a version behind, or one that
+  // answered partially, degrades the panel rather than destroying the page.
   const s = STATUS[data.status] ?? STATUS.ok;
   const Icon = s.icon;
-  const cap = data.capacity;
-  const backlog = cap.backlog;
+  const cap = data.capacity ?? EMPTY_CAPACITY;
+  const backlog = cap.backlog ?? 0;
+  const queues = Array.isArray(data.queues) ? data.queues : [];
+  const staleDevices = Array.isArray(data.stale_devices) ? data.stale_devices : [];
+  const workers = data.workers ?? { concurrency: 0, job_timeout_ms: 0, poll_interval_ms: 0 };
 
   return (
     <div className="space-y-4">
@@ -103,7 +145,7 @@ export default function PollerHealthCard() {
       {/* Capacity. headroom and backlog are shown together deliberately: headroom
           alone once reported "healthy" to an operator with 752k jobs queued. */}
       <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-6 gap-3">
-        <Stat label="Devices" value={String(cap.devices)} />
+        <Stat label="Devices" value={String(cap.devices ?? 0)} />
         <Stat
           label="Headroom"
           value={cap.headroom == null ? '—' : `${cap.headroom}×`}
@@ -120,8 +162,8 @@ export default function PollerHealthCard() {
         <Stat label="Slowest (p90)" value={cap.p90_fast_poll_ms == null ? '—' : `${cap.p90_fast_poll_ms} ms`} />
         <Stat
           label="Workers"
-          value={String(data.workers.concurrency)}
-          hint={`${data.workers.poll_interval_ms / 1000}s interval`}
+          value={String(workers.concurrency)}
+          hint={`${(workers.poll_interval_ms || 0) / 1000}s interval`}
         />
       </div>
 
@@ -148,7 +190,7 @@ export default function PollerHealthCard() {
             </tr>
           </thead>
           <tbody>
-            {data.queues.map(q => (
+            {queues.map(q => (
               <tr key={q.name} className="border-b border-gray-100 dark:border-slate-800 last:border-0">
                 <td className="py-2 pr-4 font-mono text-xs text-gray-900 dark:text-white">{q.name}</td>
                 <td className={`py-2 pr-4 text-right tabular-nums ${
@@ -195,15 +237,15 @@ export default function PollerHealthCard() {
       {/* Which devices are actually being missed */}
       <div>
         <h4 className="text-sm font-medium text-gray-900 dark:text-white mb-2">
-          Devices not polled recently {data.stale_devices.length > 0 && `(${data.stale_devices.length})`}
+          Devices not polled recently {staleDevices.length > 0 && `(${staleDevices.length})`}
         </h4>
-        {data.stale_devices.length === 0 ? (
+        {staleDevices.length === 0 ? (
           <p className="text-xs text-gray-500 dark:text-slate-400">
             Every device has been polled successfully in the last few minutes.
           </p>
         ) : (
           <ul className="space-y-1.5">
-            {data.stale_devices.map(d => (
+            {staleDevices.map(d => (
               <li key={`${d.id}-${d.kind}`} className="text-xs flex flex-wrap gap-x-2 gap-y-0.5">
                 <span className="font-medium text-gray-900 dark:text-white">{d.name}</span>
                 <span className="text-gray-500 dark:text-slate-400">