[Enhancement]: Add lightweight case-level commenting for collaboration

### Is your feature request related to a problem?

Even with case-level documentation, SimBoard still lacks a lightweight way for users to discuss case-specific context, ask questions, or leave follow-up remarks tied to a case.

Today, case pages are effectively read-only aggregation views with no built-in discussion surface. That makes collaboration harder when users need to:
- clarify case-level notes
- record review comments or follow-up questions
- capture small updates that should not overwrite canonical notes
- preserve discussion history around case interpretation or maintenance

Without a dedicated commenting surface, those conversations move to external systems or are lost entirely.

### Describe the solution you'd like

Add lightweight case-level commenting on the Case Details page.

Initial scope:
- allow authenticated, authorized users to add comments tied to a case
- show comments in chronological order on the Case Details page
- preserve author and timestamp metadata for each comment
- keep the first version simple and case-level only
- treat comments as discussion separate from canonical case notes

Recommended constraints:
- do not turn this into a full wiki or collaborative editor
- do not support attachments, rich embeds, or inline annotations in the first version
- do not expand to simulation-level or compare-page comments in this issue
- keep moderation, edit/delete policy, and notification behavior minimal unless product requires more

### Describe alternatives you've considered

Rely on case notes only.
- Good for canonical documentation, but poor fit for discussion, questions, and review history.

Bundle comments into the case notes issue.
- Increases scope substantially because commenting needs its own data model, API behavior, permissions, and UI states.

Use an external discussion system only.
- Avoids implementation work, but fragments workflow and loses SimBoard-local context.

### Additional context

This should follow case-level notes as a separate enhancement, not be combined with #226.

Comments are a different product capability from markdown notes:
- notes are canonical documentation owned by editors
- comments are conversational updates and discussion history

Keeping this separate will make implementation, testing, and product review much clearer.

Out of scope:
- threaded replies beyond a simple first version unless explicitly required
- reactions, mentions, notifications, or subscriptions
- attachments or file uploads
- simulation-level comments
- compare-page comments

No files, tests, or entry points are named in the issue. Start by locating the Case Details page and the existing case-notes and authentication paths, then trace how case data is persisted and exposed. Done means authorized users can add case-level comments, comments display chronologically with author and timestamp metadata, and discussion remains separate from canonical notes.

Written by the indexing model from the issue text.