Harden poller from Codex review: close residual storm/robustness gaps

Adversarial review (gpt-5.5) of the new architecture surfaced real gaps:

Hardware-safety / storm prevention:
- Restart-storm guard: on startup, defer the first sweep if a sweep ran
  < POLL_SWEEP_INTERVAL ago (persisted in Redis), so deploy-cycling can no
  longer trigger back-to-back full hardware sweeps (poller.py).
- Centralize pacing in the hardware gate: POLL_DRIVE_GAP is now held after
  EVERY hardware op (SMART, SES, host, MegaRAID, ledctl), not just the
  enclosure-drive loop (hwgate.py); removed the now-redundant per-loop sleeps.
- Single-instance lock made atomic (Lua compare-and-set / compare-and-expire)
  and the refresher is now fatal on any error + has a done-callback, so a
  dropped lock can never leave two pollers sweeping concurrently (store.py,
  poller.py).
- Warn loudly when POLL_CONCURRENCY > 1.

Correctness:
- Heartbeat now actually writes: cache_set omits EX when ttl<=0 (Redis
  rejects EX 0), so poller meta/last_sweep_ts persists — this also enables
  the restart-storm guard and the poller_fresh health flag (cache.py).
- Web image runs a single uvicorn worker so the MQTT publisher is a true
  singleton (two workers shared a client_id and flapped the broker) (Dockerfile).
- LED enqueue catches Redis errors -> API returns 503, not 500 (store.py).

Deploy independence:
- build.sh takes a target (web|poller|all) so web iterations never rebuild
  or repush the poller image.

Nits: drop dead SMART_CACHE_TTL constant.

services/cache.py

@@ -58,10 +58,13 @@ async def cache_get(key: str) -> Any | None:
 
 
 async def cache_set(key: str, value: Any, ttl: int = 120) -> None:
-    """SET key in Redis with expiry, silently catches errors."""
+    """SET key in Redis. ttl<=0 stores with no expiry (Redis rejects EX 0)."""
     if _redis is None:
         return
     try:
-        await _redis.set(key, json.dumps(value), ex=ttl)
+        if ttl and ttl > 0:
+            await _redis.set(key, json.dumps(value), ex=ttl)
+        else:
+            await _redis.set(key, json.dumps(value))
     except Exception as e:
         logger.warning("Redis SET %s failed: %s", key, e)