Add comments to task instance

[dodekapod]

Implementation of comments feature for task instances: users can write an optional free-text comment on a task, which auto-saves before advancing to the next instance

Details:

• new Comment model
• upsert method
• API endpoint
• new React CommentSection component with a textarea and character counter, wired into the task page lifecycle

Cleanup VotesProvider and TasksProvider by wrapping callbacks and context values in useCallback/useMemo.

[Matteovanypersele]

I reviewed and tested the "Add comments to task instance" commit (deployed locally, table created, create/edit/persistence working end-to-end UI→API→DB). It works.

One thing about back/Dockerfile: the three added COPY lines (back/app, entrypoint.sh, requirements.txt) seems redundant — line 14 (COPY back/ .) Was this intentional (e.g. to work around a Docker layer-cache
issue), or a leftover from debugging?