Split into dedicated hardware poller + read-only consumers

Rearchitect so a single poller process is the sole owner of all SAS/SMART/
SES hardware I/O, serialized behind one gate and paced, writing results to
Redis. The API/web and MQTT publisher become pure Redis readers — they no
longer issue any subprocess and can restart freely without touching the bus.

This addresses backplane/expander stress from concurrent + restart-triggered
SMART/SES storms (the prior model re-ran a hardware sweep on every container
start, and polled sg_ses 0x02+0x07 every 60s; 0x07 errored on the IOM6/
Xyratex expanders).

- poller.py: paced sweep (inventory, SMART per-drive w/ gap, SES 0x02, ZFS,
  host/MegaRAID), startup jitter, single-instance Redis lock, LED-queue worker
- services/hwgate.py: global serialization semaphore (POLL_CONCURRENCY=1)
- services/store.py: Redis as the only producer<->consumer interface + LED queue
- services/health.py: shared drive-health classifier (fixes overview double-count)
- gate smartctl/sg_ses/zpool/ledctl; drop sg_ses 0x07 from the hot path
- routers + temps read-only from the store; main.py drops the in-process poller
- compose: 3 services (privileged poller + unprivileged app + redis) w/ pacing knobs

services/health.py

@@ -0,0 +1,21 @@
+"""Shared drive-health classification.
+
+Single source of truth for turning a parsed SMART dict into a
+healthy/warning/error status, used by both the poller (host drives) and
+the API (enclosure drives) so the rule never drifts between them.
+"""
+
+
+def compute_health_status(smart: dict) -> str:
+    """Return "healthy", "warning", or "error" for a SMART dict."""
+    healthy = smart.get("smart_healthy")
+    realloc = smart.get("reallocated_sectors") or 0
+    pending = smart.get("pending_sectors") or 0
+    unc = smart.get("uncorrectable_errors") or 0
+    supported = smart.get("smart_supported", True)
+
+    if healthy is False:
+        return "error"
+    if realloc > 0 or pending > 0 or unc > 0 or (healthy is None and supported):
+        return "warning"
+    return "healthy"