GitHub App: auto-provision projects on App install (installation events)

[alexmond]

Part of #440.

Goal

When the App is installed/uninstalled on an org or repo, auto-provision/retire UniTrack projects — vs the manual import shipped in #209.

Approach

• On installation / installation_repositories webhook events (delivered by the webhook-coverage sub-task), call ProjectImportService to create projects for added repos and mark removed ones.
• Persist the installationId per repo/project so the App-auth foundation can resolve the right installation token.

Acceptance

• Installing the App on a repo auto-creates its UniTrack project.
• Removing the App retires/flags the project (no orphaned access).

Depends on

App-auth foundation (installation id/token) + the webhook event-coverage sub-task.