fix(poller): cap Redis memory, and stop retrying periodic polls (v0.23.18-beta)

The two follow-ups left open by v0.23.17. One is a real fix; the other turned
out to need explaining rather than fixing.

**Redis had no memory ceiling.** v0.23.17 stopped the poller from growing Redis
without bound, but nothing stopped anything else from doing so, and the host had
no protection either way. Redis now runs with a 512 MB cap (REDIS_MAXMEMORY).

The eviction policy is the part worth care. `allkeys-lru` would be actively
dangerous here: BullMQ job data carries no TTL, so an LRU sweep across all keys
could discard queued work with no trace. `volatile-lru` evicts only keys that
set an expiry — which is exactly the poller's caches and in-flight claims, all
of which are safe to lose because losing one merely causes an earlier re-poll.
Verified rather than assumed: sampled BullMQ job keys report ttl=-1 and poller
cache keys 528-3408s. If pressure ever exhausts the volatile keys Redis refuses
writes instead of evicting queue state, which is loud and survivable.

**The 14% fast-poll failure rate is not a bug.** Current rate across all devices
and job kinds is 0% over 1,462 attempts. The historical 230,707 failures were
genuine device downtime: device_availability records 324 outages totalling ~1,003
hours across four lab devices, which at a 30-second cadence and two attempts per
cycle predicts ~240,700 failures — within 4% of what was recorded. Nothing to
fix; retention had simply been hoarding an accurate count.

It did expose real waste, though. Periodic polls no longer retry in-job, because
the schedule already is the retry: a device that did not answer is unlikely to
answer a second later, and each extra attempt occupies a worker for up to the
job timeout. On a fleet with several devices down that doubled the capacity they
consumed — the same capacity the rest of the fleet was short of in #114. It also
doubled the recorded failure count, which is how ~1,000 hours of downtime came
to look like 231k distinct faults. User-initiated full syncs keep their retries.

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

.env.example

@@ -21,3 +21,16 @@ INFLUXDB_TOKEN=mytoken123456789
 INFLUXDB_ORG=mikrotik-manager
 INFLUXDB_BUCKET=metrics
 INFLUXDB_ADMIN_PASSWORD=admin_password_123
+
+# ─── Poller tuning (see GET /api/system/poller) ──────────────────────────────
+# Raise concurrency if `headroom` in the poller health endpoint drops near 1.0 —
+# below that the queue grows every cycle and devices go stale.
+# POLLER_CONCURRENCY=12
+# POLLER_JOB_TIMEOUT_MS=45000
+# POLLER_INTERVAL_MS=30000
+# POLLER_JOB_RETENTION_SEC=3600
+# POLLER_JOB_RETENTION_COUNT=5000
+
+# Redis memory ceiling. Only TTL-bearing keys are evicted (volatile-lru), so
+# queued jobs are never discarded; the cap exists to protect the host.
+# REDIS_MAXMEMORY=512mb