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/hwgate.py

@@ -0,0 +1,27 @@
+"""Global hardware-access gate.
+
+Every command that touches the SAS bus — smartctl, sg_ses, zpool, ledctl —
+must run inside this gate. It serializes hardware I/O so the poller can
+never fire a thundering herd of subprocesses at fragile SAS expanders
+(which on some shelves drop out and suspend pools under concurrent load).
+
+Concurrency defaults to 1 (fully serialized). It can be raised via
+POLL_CONCURRENCY for healthier backplanes, but 1 is the safe default.
+
+The gate is a per-process asyncio.Semaphore — only the poller process
+calls hardware, so a per-process gate is sufficient. The semaphore is
+created lazily on first use so it binds to the running event loop.
+"""
+import asyncio
+import os
+
+_sem: asyncio.Semaphore | None = None
+
+
+def gate() -> asyncio.Semaphore:
+    """Return the shared hardware semaphore (use as `async with gate():`)."""
+    global _sem
+    if _sem is None:
+        n = max(1, int(os.environ.get("POLL_CONCURRENCY", "1")))
+        _sem = asyncio.Semaphore(n)
+    return _sem