fix(mqtt): harden TLora MQTT observer and trim MQTT task stack use

- LilyGo T-LoRa V2.1 observer envs: set ESP32_CPU_FREQ=240 and
  MQTT_TASK_STACK_SIZE=16384 only for those targets so classic ESP32
  TLS + JWT connect fits the FreeRTOS task; other boards keep the
  default 8192 B unless overridden.
- MQTTBridge: use large JSON stack fallbacks only for PSRAM builds when
  heap-backed buffers may be null; non-PSRAM paths use the inline
  member buffers only, cutting peak stack in status/packet/raw publish.
- MQTTBridge.h: update the StaticJsonDocument comment (no longer refers
  to an 8 KiB MQTT task stack).

src/helpers/bridges/MQTTBridge.cpp

@@ -1665,8 +1665,12 @@ void MQTTBridge::publishStatusToSlot(int index) {
 
   // Reuse pre-allocated buffer to avoid heap alloc/free churn under memory pressure.
   // _status_json_buffer and _last_raw_data are both Core 0-owned; no mutex needed.
+  #if defined(BOARD_HAS_PSRAM)
   char fallback_status_buffer[STATUS_JSON_BUFFER_SIZE];
   char* json_buffer = (_status_json_buffer != nullptr) ? _status_json_buffer : fallback_status_buffer;
+  #else
+  char* json_buffer = _status_json_buffer;
+  #endif
 
   char origin_id[65];
   char timestamp[32];
@@ -2356,8 +2360,12 @@ bool MQTTBridge::publishStatus() {
 
   // Reuse pre-allocated buffer to avoid heap alloc/free churn under memory pressure.
   // _status_json_buffer and _last_raw_data are both Core 0-owned; no mutex needed.
+  #if defined(BOARD_HAS_PSRAM)
   char fallback_status_buffer[STATUS_JSON_BUFFER_SIZE];
   char* json_buffer = (_status_json_buffer != nullptr) ? _status_json_buffer : fallback_status_buffer;
+  #else
+  char* json_buffer = _status_json_buffer;
+  #endif
   char origin_id[65];
   char timestamp[32];
   char radio_info[64];
@@ -2475,7 +2483,8 @@ bool MQTTBridge::publishPacket(mesh::Packet* packet, bool is_tx,
   }
   #endif
 
-  // Use pre-allocated buffer; fallback to single stack buffer if not available
+  // Use pre-allocated buffer; stack fallback only when PSRAM heap alloc may be null.
+#if defined(BOARD_HAS_PSRAM)
   char json_buffer_stack[PUBLISH_JSON_BUFFER_SIZE];
   char* active_buffer;
   size_t active_buffer_size;
@@ -2486,6 +2495,10 @@ bool MQTTBridge::publishPacket(mesh::Packet* packet, bool is_tx,
     active_buffer = json_buffer_stack;
     active_buffer_size = PUBLISH_JSON_BUFFER_SIZE;
   }
+#else
+  char* active_buffer = _publish_json_buffer;
+  const size_t active_buffer_size = PUBLISH_JSON_BUFFER_SIZE;
+#endif
   char origin_id[65];
 
   strncpy(origin_id, _device_id, sizeof(origin_id) - 1);
@@ -2552,7 +2565,7 @@ bool MQTTBridge::publishRaw(mesh::Packet* packet) {
 
   refreshOriginFromPrefs();
 
-  // Use pre-allocated buffer; fallback to single stack buffer if not available
+#if defined(BOARD_HAS_PSRAM)
   char json_buffer_stack[PUBLISH_JSON_BUFFER_SIZE];
   char* active_buffer;
   size_t active_buffer_size;
@@ -2563,6 +2576,10 @@ bool MQTTBridge::publishRaw(mesh::Packet* packet) {
     active_buffer = json_buffer_stack;
     active_buffer_size = PUBLISH_JSON_BUFFER_SIZE;
   }
+#else
+  char* active_buffer = _publish_json_buffer;
+  const size_t active_buffer_size = PUBLISH_JSON_BUFFER_SIZE;
+#endif
   char origin_id[65];
 
   strncpy(origin_id, _device_id, sizeof(origin_id) - 1);

src/helpers/bridges/MQTTBridge.h

@@ -223,7 +223,7 @@ private:
   char _status_json_buffer[STATUS_JSON_BUFFER_SIZE];
   #endif
 
-  // JSON document scratch space — inline StaticJsonDocument keeps the pool off the 8 KB MQTT
+  // JSON document scratch space — inline StaticJsonDocument keeps the pool off the MQTT
   // task stack and eliminates two separate heap allocations (fragmentation reduction).
   StaticJsonDocument<PUBLISH_JSON_BUFFER_SIZE> _packet_json_doc;
   StaticJsonDocument<STATUS_JSON_BUFFER_SIZE>  _status_json_doc;

variants/lilygo_tlora_v2_1/platformio.ini

@@ -155,6 +155,8 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
   -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
+  -D MQTT_TASK_STACK_SIZE=16384
+  -D ESP32_CPU_FREQ=240
 ;  -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1
@@ -197,6 +199,8 @@ build_flags =
   -D WITH_MQTT_BRIDGE=1
   -D MAX_MQTT_BROKERS=3
   -D MQTT_MAX_PACKET_SIZE=1024
+  -D MQTT_TASK_STACK_SIZE=16384
+  -D ESP32_CPU_FREQ=240
   -D MQTT_DEBUG=1
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1