Feat/attachments links epics burndown

[nazarli-shabnam]

This pull request introduces new handler implementations and corresponding tests for epics, attachments, and cycle progress in the API. It adds full CRUD and linking endpoints for epics, provides endpoints for managing issue attachments, and introduces endpoints and tests for cycle burndown/progress analytics. These changes significantly expand the API's coverage and testability for key project management features.

New API Handlers

• Epic endpoints:
  • Implements EpicHandler with endpoints to list, create, retrieve, update, and delete epics, as well as manage their child issues and add issues to epics. (api/internal/handler/epic.go)
• Attachment endpoints:
  • Adds AttachmentHandler with endpoints to initiate, confirm, list, and delete file attachments for issues. (api/internal/handler/attachment.go)
• Cycle progress endpoints:
  • Adds new endpoints to retrieve cycle progress (burndown data) and analytics, including error handling for authentication and permissions. (api/internal/handler/cycle.go)

Test Coverage

• Epic handler tests:
  • Adds comprehensive tests for epic CRUD operations, issue linking, and link management, including authentication and permission checks. (api/internal/handler/epic_test.go)
• Cycle progress tests:
  • Adds tests for cycle progress and analytics endpoints, covering authentication, empty cycles, and permission scenarios. (api/internal/handler/cycle_progress_test.go)

[Copilot]

Pull request overview

Adds Epic support to issues and introduces GORM models for issue links and attachments, aligning the API model layer with the issue-tracking schema.

Changes:

• Adds is_epic boolean column + partial index via a new migration.
• Extends Issue model with IsEpic.
• Introduces IssueLink, FileAsset, and IssueAttachment model types.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

File	Description
api/migrations/000004_epics.up.sql	Adds is_epic column and partial index for epic filtering.
api/migrations/000004_epics.down.sql	Rolls back the epic column/index changes.
api/internal/model/issue.go	Adds IsEpic and new models for links/assets/attachments.

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