feat: add shared project deployment with Dokploy project-level variables

- Add SHARED_PROJECT_ID and SHARED_ENVIRONMENT_ID to all docker-compose files
- Use Dokploy's project-level variable syntax: ${{project.VARIABLE}}
- Deploy all user AI stacks to a single shared Dokploy project
- Update DOKPLOY_DEPLOYMENT.md with shared project configuration guide
- Add comprehensive SHARED_PROJECT_DEPLOYMENT.md architecture documentation

Benefits:
- Centralized management (all stacks in one project)
- Resource efficiency (no per-user project overhead)
- Simplified configuration (project-level shared vars)
- Better organization (500 apps in 1 project vs 500 projects)

How it works:
1. Portal reads SHARED_PROJECT_ID from environment
2. Docker-compose uses ${{project.SHARED_PROJECT_ID}} to reference project-level vars
3. Dokploy resolves these at runtime
4. Portal deploys user stacks as applications within the shared project

Fallback: If variables not set, falls back to legacy behavior (separate project per user)

docker-compose.prod.yml

@@ -15,6 +15,8 @@ services:
       - STACK_DOMAIN_SUFFIX=${STACK_DOMAIN_SUFFIX:-ai.flexinit.nl}
       - STACK_IMAGE=${STACK_IMAGE:-git.app.flexinit.nl/flexinit/agent-stack:latest}
       - RESERVED_NAMES=${RESERVED_NAMES:-admin,api,www,root,system,test,demo,portal}
+      - SHARED_PROJECT_ID=${{project.SHARED_PROJECT_ID}}
+      - SHARED_ENVIRONMENT_ID=${{project.SHARED_ENVIRONMENT_ID}}
     env_file:
       - .env
     restart: unless-stopped