[FEATURE] Per-Project Export Template Assignment and Default Selection

[therealbrad]

Feature Summary

Allow admins to assign specific Case Export Templates to individual projects and set a per-project default template. This enables different projects to surface only the templates relevant to their tech stack.

Problem Statement

Current Situation

With the initial Templated Export feature (#83), all enabled export templates are available globally across every project. A team using Playwright sees Cypress and WebdriverIO templates they'll never use, and vice versa.

Desired Outcome

Project admins can configure which export templates appear for their project and designate a project-specific default. When users export test cases, they only see the templates assigned to that project, with the project default pre-selected.

Proposed Solution

Schema Changes

• New join model CaseExportTemplateProjectAssignment linking CaseExportTemplate to Project (follows the existing TemplateProjectAssignment pattern for case field templates)
• Add a defaultCaseExportTemplate relation on Project (or a boolean flag on the assignment) to track which template is the project default

Admin UI

• In the project settings or admin area, add a section to manage which export templates are assigned to the project
• Allow selecting a default template per project
• If no templates are explicitly assigned, fall back to showing all enabled templates (backward compatible)

Export Dialog

• Filter the template dropdown to only show templates assigned to the current project
• Pre-select the project's default template (falling back to the global default if none is set)

Acceptance Criteria

• Many-to-many assignment between CaseExportTemplate and Project via a join model
• Admin UI to assign/unassign export templates to a project
• Ability to set a default export template per project
• Templated Export dialog only shows templates assigned to the current project
• If no templates are assigned to a project, all enabled templates are shown (backward compatible)
• Project default template is pre-selected in the export dialog

Related Issues

• Extracted from [FEATURE] Templated Export for Test Cases #83 (Templated Export for Test Cases) — deferred from v1 scope
• Follows the existing pattern of TemplateProjectAssignment for case field templates

Implementation Effort

• Small (1-2 days)
• Medium (3-5 days)
• Large (1-2 weeks)
• Extra Large (>2 weeks)