chore(mqtt): silence periodic stats log for production; add get mqtt.stats CLI

The 30s "MQTT: Memory" line was useful during the outbox/sync-publish
investigation but is spam for production. Gate the periodic logMemoryStatus()
call in the MQTT task loop behind MQTT_MEMORY_DEBUG (a dedicated diagnostics
flag, not enabled by plain MQTT_DEBUG or production builds) and stop the
heltec_v3 variant from force-enabling MQTT_MEMORY_DEBUG on its observer_mqtt
envs (now commented out to match heltec_v4). logMemoryStatus() itself is kept
intact for opt-in debugging.

Expose the same data on demand via a new `get mqtt.stats` CLI command backed by
MQTTBridge::formatMqttStatsReply(): free/max heap, queue depth, outbox total,
and per-slot publish ok/err counts (1-based, matching the msgs: line). Fits the
160-byte reply buffer at 6 slots; returns "(bridge not running)" when down.

src/helpers/CommonCLI_Observer.cpp

@@ -695,6 +695,8 @@ bool CommonCLI::handleObserverGetCmd(uint32_t sender_timestamp, const char* conf
       start = (int)_atoi(start_arg);
     }
     formatMQTTPresetListReply(reply, 160, start);
+  } else if (memcmp(config, "mqtt.stats", 10) == 0) {
+    MQTTBridge::formatMqttStatsReply(reply, 160);
   } else if (memcmp(config, "mqtt.status", 11) == 0) {
     MQTTBridge::formatMqttStatusReply(reply, 160, &_mqtt_prefs);
   } else if (memcmp(config, "mqtt.packets", 12) == 0) {