Add public MQTT broker URL configuration for email templates

The MQTT broker URL in credential emails was using the internal Docker
address (mqtt://mosquitto:1883) which is not accessible from external
OwnTracks apps. Added MQTT_PUBLIC_BROKER_URL environment variable to
configure the publicly accessible broker address for client apps.

Changes:
- Add MQTT_PUBLIC_BROKER_URL to .env.example with documentation
- Update send-credentials route to use public URL with fallback
- Maintain backward compatibility with existing MQTT_BROKER_URL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -29,12 +29,18 @@ ENCRYPTION_KEY=your-32-byte-hex-key-here
 # MQTT Configuration
 # ======================================
 
-# MQTT Broker URL
+# MQTT Broker URL (internal - for app container to connect)
 # Development (local): mqtt://localhost:1883
 # Docker Compose: mqtt://mosquitto:1883
 # Production: mqtt://your-mqtt-broker:1883
 MQTT_BROKER_URL=mqtt://mosquitto:1883
 
+# MQTT Public Broker URL (external - for OwnTracks apps to connect)
+# This is the publicly accessible address that will be sent in emails
+# Development (local): mqtt://localhost:1883
+# Production: mqtt://your-public-domain.com:1883 or mqtt://your-server-ip:1883
+MQTT_PUBLIC_BROKER_URL=mqtt://192.168.10.118:1883
+
 # MQTT Admin Credentials (für MQTT Subscriber und Password File Generation)
 MQTT_ADMIN_USERNAME=admin
 MQTT_ADMIN_PASSWORD=admin