infra: tune flash params + persist zigbee network key for paired devices

- FLASH_INTERVAL_SECONDS 3 -> 2 (user preference)
- Add network_key / pan_id / ext_pan_id to zigbee2mqtt configuration
  so paired devices can rejoin after a config wipe. Network key
  preserved from previous working state on RPi.
- Doorbell listener flash logic with even toggle count so plug ends
  at starting state

infra/zigbee2mqtt/configuration.yaml

@@ -9,6 +9,34 @@ serial:
   rtscts: false
 advanced:
   log_level: debug
+  network_key:
+    - 146
+    - 30
+    - 124
+    - 1
+    - 70
+    - 32
+    - 235
+    - 196
+    - 27
+    - 113
+    - 122
+    - 230
+    - 7
+    - 177
+    - 134
+    - 45
+  pan_id: 3926
+  ext_pan_id:
+    - 225
+    - 105
+    - 42
+    - 213
+    - 37
+    - 25
+    - 93
+    - 235
+
 frontend:
   enabled: true
   port: 8080