fix(webhook): use installation_id in uninstall cleanup

[leno23]

Summary

• Fix installation.deleted cleanup to filter by the actual database column name installation_id.
• Keep the existing soft-clear behavior: set installationId to null and registered to false for repos owned by the deleted installation.

Related Issues

Closes #92

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• npm ci
• npm run build
• npm run lint
• npm run format:check
• git diff --check HEAD~1 HEAD

Note: this repository does not currently define a test script/test runner in packages/das/package.json, so I validated with the available build, lint, formatting, and whitespace gates.

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

AI assistance disclosure

This draft PR was prepared with AI assistance. The change is intentionally limited to the uninstall cleanup query column name and was validated with the commands above.

[anderdc]

why is this marked as a draft?