fix(webhook): reconcile repository renames by GitHub id

[volcano303]

Summary

• Add repos.github_repo_id so the mirror can identify repositories by GitHub's stable repository ID.
• Reconcile webhook payloads whose repository.full_name changed by moving existing repo-keyed mirror rows to the new full name before handler dispatch.
• Route installation add/remove events through the same identity path so future payloads keep the stable repo ID populated.

Related Issues

Fixes #72

Type of Change

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

Testing

• npm run format:check
• npm run lint
• npm run build
• git diff --check

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

[anderdc]

Not pursuing — repository renames on subnet-tracked repos are vanishingly rare, and if one ever does happen an operator can repair the rows with a one-shot SQL UPDATE per repo-keyed table in less time than reviewing this PR. The cost of automatic reconciliation in the webhook hot path (extra SELECT per event, schema change, +182 lines on the trust surface) outweighs the cost of manual repair on the unlikely day it triggers. Closing.