fix(backend): warn on missing GitHub App org installation

[brendan-kellam]

Fixes SOU-1588

Summary

• replace createOctokitFromToken and getOctokitWithGithubApp with a single createOctokit factory
• prefer GitHub App authentication for owner-scoped requests and fall back to token authentication when no app is configured
• warn and skip only an organization without an app installation while continuing to sync valid organizations
• keep initialization, licensing, and token-generation failures fatal

Testing

• yarn workspace @sourcebot/backend test
• yarn workspace @sourcebot/backend build

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