feat(discussion): add cohort-scoped peer discussion forum

[Umesh-369]

What

Adds a Discussion Forum section to the course sidebar, letting students
in the same course + cohort ask and answer each other's questions.
Instructors/admins can view and moderate (pin/delete).

Why

Students currently have no in-app way to get peer help on doubts tied
to their specific cohort. This adds a lightweight, scoped Q&A space
without touching proctoring, quizzes, or grading.

Scope

• New backend module: thread + reply CRUD, cohort-membership enforced
  server-side on every read and write.
• New frontend section: sidebar entry, thread list, thread detail,
  new-thread composer — rendered in the existing course main panel,
  same color palette/components as the rest of the course UI.

Explicitly out of scope (v1)

Upvotes, nested replies, attachments, notifications, forum search,
accepted-answer marking — noted as follow-ups.

How to test

1. vibe start frontend backend
2. Log in as a student enrolled in [course]/[cohort A]; open the course,
   click Discussion Forum, create a thread, reply from a second account
   in the same cohort.
3. Log in as a student in [cohort B] of the same course; confirm you do
   NOT see cohort A's threads.
4. Log in as the course instructor; confirm threads from both cohorts
   are visible and you can delete a thread.