Refactor billing API and enhance subgraph management documentation

[jfrench9]

Summary

This PR modernizes the billing API by replacing deprecated payment method management with a streamlined portal session approach, while enhancing subgraph creation and deletion documentation with clearer tier requirements and backup location details.

Key Accomplishments

Billing API Modernization

• Replaced deprecated payment method management with a new portal session system that provides a more user-friendly billing interface
• Removed legacy subscription cancellation functionality that was no longer aligned with the backend API
• Streamlined billing workflow by consolidating payment operations through the portal session approach

Enhanced Subgraph Management

• Updated subgraph creation documentation to clearly specify tier requirements, helping users understand prerequisites before attempting to create subgraphs
• Improved delete subgraph documentation with specific backup location details, ensuring users know where their data is stored after deletion

Model Improvements

• Added new graph_subscription_response and portal_session_response models to support the updated API structure
• Enhanced list_subgraphs_response model with additional functionality
• Simplified create_subgraph_request model by removing unnecessary complexity

Breaking Changes

⚠️ This PR introduces breaking changes:

• Removed API endpoints: cancel_subscription and update_org_payment_method are no longer available
• Removed models: cancellation_response and update_payment_method_response have been deleted
• Renamed functionality: Payment method updates are now handled through the portal session system

Testing Notes

• Verify that the new portal session creation works correctly for billing operations
• Test subgraph creation with the updated tier requirement validation
• Confirm that subgraph deletion provides accurate backup location information
• Ensure all model imports and exports are functioning correctly after the restructuring

Infrastructure Considerations

• Applications using the removed billing and subscription endpoints will need to be updated to use the new portal session approach
• The reduced codebase (net reduction of 328 lines) should improve maintainability and reduce technical debt
• Monitor for any integration issues with systems that previously relied on the deprecated endpoints

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🤖 Generated with Claude Code

Branch Info:

• Source: chore/backend-api-alignment
• Target: main
• Type: feature

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