doorbell-listener: flash plug on alert (configurable interval/duration)

On a fresh (< MAX_AGE_SECONDS) ntfy message, spawn a thread that
publishes N TOGGLEs spaced FLASH_INTERVAL_SECONDS apart so total
runtime is FLASH_DURATION_SECONDS. Defaults: 5 toggles @ 3s = 15s.

Env vars (all in compose.yaml):
  FLASH_INTERVAL_SECONDS  default 3
  FLASH_DURATION_SECONDS  default 15
  MAX_AGE_SECONDS         default 180 (replay protection)

Each alert gets its own thread; concurrent alerts spawn concurrent
flashes. z2m just flips state per toggle, so the lamp alternates
regardless of overlap.

infra/compose.yaml

@@ -57,6 +57,8 @@ services:
   # Listens to ntfy.sh for doorbell events and toggles a smart plug on each alert.
   # Topic: ALERT_klubhaus_topic_test (matches ESP32 firmware DEBUG_MODE suffix)
   # Action: TOGGLE on zigbee2mqtt/Sideboard Lamp/set
+  # On a fresh alert, flash the plug: N toggles spaced FLASH_INTERVAL_SECONDS
+  # so the total runtime is FLASH_DURATION_SECONDS.
   doorbell-listener:
     build: ./doorbell-listener
     container_name: doorbell-listener
@@ -70,6 +72,9 @@ services:
       MQTT_PORT: "1883"
       PLUG_TOPIC: "zigbee2mqtt/Sideboard Lamp/set"
       POLL_INTERVAL: "30"
+      MAX_AGE_SECONDS: "180"
+      FLASH_INTERVAL_SECONDS: "3"
+      FLASH_DURATION_SECONDS: "15"
       TZ: America/Los_Angeles