Per @lunny's feedback, rework to reuse the existing Comment table
instead of a standalone commit_comment table. The junction table
(commit_comment) now only stores repo_id, commit_sha, comment_id.
Actual comment data (content, tree_path, line, patch, poster) lives
in the Comment table with Type = CommentTypeCommitComment (39).
This gives commit comments reactions, attachments, and all existing
comment infrastructure for free.
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
