Summary

This PR implements inline code comments on individual commits, similar to the existing PR code review functionality.

Features

• ✅ Add inline comments on any line of a commit diff
• ✅ Edit and delete your own comments
• ✅ Attachment support
• ✅ @mention notifications
• ✅ Works in both unified and split diff views

Implementation Details

• Reuses existing Comment model infrastructure (CommitSHA, Line, TreePath already supported)
• New service layer for commit comment CRUD operations
• API routes: /commit/:sha/comments for create/update/delete
• Templates updated to show comment UI when PageIsCommit is true
• Existing PR comment JavaScript handles commit comments generically

Testing

• ✅ Builds successfully without errors
• Backend implementation follows existing PR comment patterns
• Reuses PR comment templates and JavaScript

Fixes #4898

/claim #4898