Add pre-deployment IP configuration guide for network transitions

- Add HOST_IP field to .env.example with documentation
- Improve TRUSTED_PROXIES comments with examples
- Create PRE_DEPLOYMENT_IP_CONFIGURATION.md guide
- Update deployment docs with network transition workflow
- Add comprehensive IP configuration checklist

MASTER_DEPLOYMENT_PLAN.md

@@ -27,6 +27,19 @@ OVERALL: 95% READY FOR PRODUCTION
 
 ## 🚀 Five-Minute Deployment
 
+### Step 0: Configure Target IP (If deploying on different network)
+
+**Special case**: If your host will be on a different IP after deployment/restart:
+
+```bash
+# See: PRE_DEPLOYMENT_IP_CONFIGURATION.md for detailed instructions
+# Quick version:
+TARGET_IP=192.168.0.121      # What IP will host have AFTER deployment?
+TARGET_DOMAIN=digiserver.local  # Optional domain name
+```
+
+This must be set in `.env` BEFORE running `docker-compose up -d`
+
 ### Step 1: Prepare (2 minutes)
 ```bash
 cd /opt/digiserver-v2