feat: add custom fields backend models, API, and tests

[Vansh5632]

Implement backend support for custom fields (work item properties) to allow projects to define and manage custom metadata for issues.

Description

• Add IssueProperty and IssuePropertyValue models
• Add migration 0113 with unique constraints
• Add ViewSets for property CRUD and bulk operations
• Add serializers with type-specific validation
• Add 30 tests (8 model, 10 serializer, 12 API) - all passing
• Support 6 field types: text, number, date, boolean, select, multi_select

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Feature (non-breaking change which adds functionality)
• [ ] Improvement (change that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Performance improvements
• [ ] Documentation update

Test Scenarios

test cases are already addded

References

Linked to issue: #8157

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[!NOTE] Adds backend support for project-defined custom fields and per-issue values with strict validation and atomic writes.

• New models IssueProperty and IssuePropertyValue with migration 0113_custom_fields_issue_property.py (unique constraints, indexes)
• Serializers: IssuePropertySerializer, IssuePropertyValueSerializer (type-validated: text/number/date/boolean/select/multi_select), plus IssuePropertyLiteSerializer
• Issue flow integration: IssueCreateSerializer/update() handle custom_fields atomically; IssueDetailSerializer.to_representation() returns custom_fields
• APIs/Views: IssuePropertyViewSet (project-level definitions CRUD), IssuePropertyValueViewSet (issue values list/create/update/delete), BulkIssuePropertyValueEndpoint (bulk get/set with transaction)
• Routes added under apps/api/plane/api/urls/project.py and apps/api/plane/app/urls/issue.py for /properties/, /issues/<issue_id>/property-values/, and /issues/<issue_id>/custom-fields/
• Tests: unit and contract coverage for models, serializers, endpoints, and soft-delete filtering

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Summary by CodeRabbit

• New Features

  • Added project-level custom field definitions (create/edit/delete) and per-issue custom field values
  • Added bulk endpoints to set and retrieve multiple custom field values on an issue
  • Custom fields now surface in issue details and activity

• Improvements

  • Stronger validation for custom field types/options and atomic (all-or-nothing) save behavior with clear error responses

• Tests

  • Added unit and integration tests covering model, serializer, API, bulk ops, validation, and edge cases

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