refactor: move installation repositories to subcollection (Firestore 1 MiB limit)

[mschilling]

Context

GitHubInstallation.repositories is stored as an inline array on the installation document. For large GitHub orgs with hundreds or thousands of repos, this can exceed Firestore's 1 MiB per-document limit, causing silent write failures.

Identified in multi-model AI review of PR #452.

Current model

installations/{id}
  repositories: [{ id, name, fullName, private }, ...]  // unbounded array

Proposed model

installations/{id}
  repositoryCount: number
  // ... other fields

installations/{id}/repos/{repoId}
  name: string
  fullName: string
  private: boolean

Impact

• Not urgent for early adoption (personal accounts have <50 repos typically)
• Becomes relevant for org-wide installations at Framna
• Requires changes to: types, repository, event handler, API controller, frontend service

Related

• PR feat: GitHub App self-service repository management #452 — GitHub App installation support
• Issue feat: track GitHub App installation ID in webhook processing #453 — installation tracking in webhook pipeline