Add work_item_custom_type_id FK to work_item_custom_* tables

Issue: [BE] Add custom_work_item_type_id column to cus... (#581951 - closed)

What does this MR do and why?

Updates existing related tables (for custom fields, lifecycles, and status mappings) to work with both the original system-defined types and these new custom types. This means users can now create personalized work item types that fit their specific workflow needs while maintaining all the existing functionality like custom fields and status tracking.

Add custom work item type migration and model

• Adds indexes for fields
• Adds fk contraints for organization_id and namespace_id

Changelog: added

Acceptance Criteria

• add new fk column work_item_custom_type_id to work_item_type_custom_fields
• add new fk column work_item_custom_type_id to work_item_type_custom_lifecycles
• add new fk column work_item_custom_type_id to work_item_custom_status_mappings
• add check constraint to ensure either system defined work item type (work_item_type_id) or custom work item (work_item_custom_type_id) is linked
• update model associations/validations (work_item_type_id or work_item_custom_type_id)

References

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Before	After

How to set up and validate locally

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