refactor: move comments router create logic to a repository

[coderabbitai]

The commentsRouter.create mutation contains too much inline logic and should delegate to a repository.

File: src/server/api/routers/listings/comments.ts line 27

Action required:

• Create a CommentsRepository (or extend ListingsRepository) to encapsulate comment creation logic.
• Update the router to call the repository method.

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Requested by @Producdevity — related to PR #375