Restructure backend into separate Django applications

[dodo920306]

Summary

Restructure the backend codebase into independent Django applications to improve modularity and maintainability.

Changes

New Applications

• user: User management module
• auth: Authentication and authorization module
• organization: Organization management module
• agent: Agent management module (later merged into node)
• node: Node management module
• channel: Channel management module
• chaincode: Chaincode management module

Code Optimization

• Replace envsubst with pure Python implementation
• Reduce duplicate code in paginators and serializers
• Remove duplicate configuration assignments
• Complete Node container creation functionality

Removed Items

• Remove legacy api/routes/ structure
• Remove unused agent-related code

Migration Notes

• Run python manage.py migrate to apply new database schema
• API endpoint paths may have changed, refer to api_engine/urls.py

Dependencies

• Depends on: Remove Celery and its related tasks #741

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🤖 This PR description was generated with AI assistance (Claude, Anthropic).

[dodo920306]

This is draft only because #741 hasn't been merged yet.