feat: complete variable management implementation and update documentation

- Update remaining Docker Compose templates with centralized variables
- Fix service tag isolation to deploy individual services only
- Update all README files with variable management architecture
- Document variable hierarchy in DEPLOYMENT_LEARNINGS.md
- Add comprehensive variable usage patterns to CLAUDE.md
- Standardize domain references using {{ subdomains.* }} pattern
- Replace hardcoded network names with {{ docker.network_name }}
- Update hairpinning configuration to use variables

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

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

roles/docker/templates/paperlessngx.env.j2

@@ -24,7 +24,7 @@
 
 # This is required if you will be exposing Paperless-ngx on a public domain
 # (if doing so please consider security measures such as reverse proxy)
-PAPERLESS_URL=https://{{ subdomains.papers }}
+PAPERLESS_URL=https://{{ subdomains.paper }}
 
 # Adjust this key if you plan to make paperless available publicly. It should
 # be a very long sequence of random characters. You don't need to remember it.