refactor: Use external Docker named volume for data persistence

Changes:
- Replace bind mount (./data) with external named volume (nlcc-data)
- Volume must be created before first run: docker volume create nlcc-data
- Improves portability and follows Docker best practices
- Better separation between code and data

Benefits:
- Data persists across container rebuilds and updates
- Easier backup and restore operations
- Platform-agnostic (works same on Linux/Windows/macOS)
- Managed by Docker's volume system
- No permission issues with bind mounts

README Updates:
- Added volume creation step to installation instructions
- Documented volume management commands (create, inspect, backup, restore)
- Added backup/restore examples using alpine container
- Clarified data persistence behavior

Note: Existing deployments using ./data bind mount will need to:
1. Backup existing data: cp -r ./data ./data-backup
2. Create volume: docker volume create nlcc-data
3. Restart container: docker-compose up -d
4. Copy data to volume if needed

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

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

README.md

@@ -87,7 +87,12 @@ cd nlcc-itinerary
    - Configure email settings (required for password resets)
    - Optionally set `ADMIN_USERNAME` and `ADMIN_PASSWORD` (otherwise auto-generated)
 
-3. Build and run with Docker Compose:
+3. Create the external Docker volume for data persistence:
+```bash
+docker volume create nlcc-data
+```
+
+4. Build and run with Docker Compose:
 ```bash
 docker-compose up -d --build
 ```
@@ -310,7 +315,29 @@ docker exec -it nlcc-itinerary sh
 
 ## Data Persistence
 
-The SQLite database is stored in the `./data` directory, which is mounted as a volume in Docker. This ensures sermon data persists across container restarts.
+The SQLite database is stored in a Docker named volume (`nlcc-data`) which is mounted to `/app/data` in the container. This ensures:
+- Data persists across container restarts and rebuilds
+- Better portability across different environments
+- Easier backup and restore operations
+- Managed by Docker volume system
+
+**Volume Management Commands**:
+```bash
+# Create the volume (required before first run)
+docker volume create nlcc-data
+
+# Inspect volume location and details
+docker volume inspect nlcc-data
+
+# Backup the database
+docker run --rm -v nlcc-data:/data -v $(pwd):/backup alpine tar czf /backup/nlcc-backup.tar.gz -C /data .
+
+# Restore from backup
+docker run --rm -v nlcc-data:/data -v $(pwd):/backup alpine tar xzf /backup/nlcc-backup.tar.gz -C /data
+
+# Remove volume (WARNING: deletes all data!)
+docker volume rm nlcc-data
+```
 
 ## License