feat(backend): add GitLab provider support

[oshuej198]

Summary

• add GitLab.com and self-managed GitLab as an SCM provider using access tokens or OAuth
• support repository discovery, nested namespaces, branches, commits, merge requests, comments, tags, and GitLab Releases
• automatically link commits, branches, and merge requests to work items by item key
• support creating and attaching milestone releases, including existing Git tags and GitLab Releases
• add validated GitLab webhooks for push, tag, merge request, note, and release events
• document GitLab token types and recommend Project or Group Access Tokens for server-to-server integrations

Dependency / review note

Important

This branch includes changes from feat/224-shared-navigation-sidebars. Please merge #224 first, then review and retest this PR after #224 has landed. Until then, the diff also contains unrelated sidebar changes from that branch.

Per the discussion in #237, this should also wait until after the 0.8.8 release before merging.

Testing

• go test ./internal/handlers ./internal/scm
• npm run check
• npm run typecheck
• manually verified against self-managed GitLab using an access token:
  • project discovery and workspace repository linking
  • default-branch commit synchronization
  • automatic linking of a DE-1 commit to the corresponding Windshift item

Closes #237

[github-actions]

This PR has merge conflicts that need to be resolved before it can be merged. Please rebase on the latest main branch.

[github-actions]

Merge conflicts have been resolved.

[github-actions]

This PR has merge conflicts that need to be resolved before it can be merged. Please rebase on the latest main branch.