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>

backend/src/services/PollerService.ts

@@ -159,12 +159,19 @@ export class PollerService {
         attempts: 3,
         backoff: { type: 'exponential', delay: 5000 },
       });
+    // Periodic polls are not retried in-job: the schedule is the retry. A device
+    // that just failed to 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 doubles the capacity they waste, which is exactly
+    // the capacity the rest of the fleet is short of (#114). Historically this
+    // also doubled the recorded failure count: 231k failed jobs on a four-device
+    // fleet turned out to be ~1,000 hours of genuine downtime, counted twice.
     } else if (type === 'fast') {
-      await this.fastQueue.add('device-fast-poll', jobData, { attempts: 2 });
+      await this.fastQueue.add('device-fast-poll', jobData, { attempts: 1 });
     } else if (type === 'slow') {
-      await this.slowQueue.add('device-slow-poll', jobData, { attempts: 2 });
+      await this.slowQueue.add('device-slow-poll', jobData, { attempts: 1 });
     } else if (type === 'logs') {
-      await this.logsQueue.add('device-logs-poll', jobData, { attempts: 2 });
+      await this.logsQueue.add('device-logs-poll', jobData, { attempts: 1 });
     } else if (type === 'macscan') {
       await this.fastQueue.add('device-macscan', jobData, { attempts: 1 });
     } else if (type === 'spectral') {