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/cache.py

@@ -38,6 +38,11 @@ def redis_available() -> bool:
     return _redis is not None
 
 
+def get_client() -> "redis.Redis | None":
+    """Return the raw Redis client for primitives not covered by get/set."""
+    return _redis
+
+
 async def cache_get(key: str) -> Any | None:
     """GET key from Redis, return deserialized value or None on miss/error."""
     if _redis is None: