fix(installation): replace find-then-insert with atomic upsert (#110)

[enjoyandlove]

Summary

Replace the check-then-insert/update pattern in InstallationHandler with a single atomic upsert. The previous implementation issued a findOneBy followed by either insert or update, which meant two round-trips and a potential race condition if the same installation event fired concurrently. The new approach uses a createQueryBuilder upsert (.orUpdate) that inserts on first encounter — setting addedAt — and only updates installationId on conflict, so addedAt is never overwritten on re-fires of installation.created or installation_repositories.added.

Related Issues

Fixes #110

Type of Change

• Bug fix

Testing

• The handler processes installation.created and installation_repositories.added events without error.
• Firing the same event twice leaves addedAt unchanged and updates installationId correctly.
• No new unit tests were added; the fix is a direct query rewrite with equivalent observable behaviour.

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added