fix(pskr): backfill pods opt out of MQTT client by default

Both `prop` and `prop-backfill` deployments use `app=prop` for the
libcluster selector, so they share a :global namespace. During a
rollout the backfill pod can win FCFS leader election and end up
running the real-time MQTT client on the same pod that's churning
through batch enrichment jobs. Worse, the backfill pod was still
running the pre-IPv4-fix image and surfacing :einval on every
30-second retry.

Gate `pskr_mqtt_enabled` off when PROP_ROLE=backfill. Explicit
PSKR_MQTT_ENABLED=true still wins if you ever want to flip it on
for a dedicated MQTT pod.

config/runtime.exs

@@ -383,12 +383,24 @@ if config_env() == :prod do
   # cluster-elected leader actually connects (`:global` registration
   # in `Microwaveprop.Pskr.Client`). Set `PSKR_MQTT_ENABLED=false`
   # to kill the entire feature without code change.
+  #
+  # Backfill pods opt out unconditionally — they share the libcluster
+  # `app=prop` selector with the hot pods, so without this gate the
+  # backfill pod can win :global election (FCFS during a rollout)
+  # and pin the MQTT client onto a node that's also under heavy
+  # batch-job load. MQTT is real-time; backfill is bursty CPU; the
+  # two don't mix on the same pod.
+  pskr_default = prop_role != "backfill"
+
   case System.get_env("PSKR_MQTT_ENABLED") do
     disabled when disabled in ["0", "false", "FALSE"] ->
       config :microwaveprop, :pskr_mqtt_enabled, false
 
-    _ ->
+    enabled when enabled in ["1", "true", "TRUE"] ->
       config :microwaveprop, :pskr_mqtt_enabled, true
+
+    _ ->
+      config :microwaveprop, :pskr_mqtt_enabled, pskr_default
   end
 
   # Optional secondary repo for read-only access to aprs.me's database.