Add a new commit_comment table and full CRUD flow to support inline
comments on commit diff views, similar to PR review comments but
standalone (no issue/PR required).
Changes:
- New CommitComment model with migration (v326)
- Web handlers for rendering form, creating, and deleting comments
- Diff context patch generation for comment positioning
- Templates for commit comment conversation, individual comments, form
- Modified diff section templates to render existing commit comments
- Reuses existing JS for add-code-comment and delete-comment flows
Closes#4898