M3: Post Model & Draft Workflow Base

Deliverables:
- Post model (title, body, tags, status, type, audience, scheduled time)
- POST /posts/ (create draft)
- PATCH /posts/:id (edit draft)
- GET /posts/feed (personalised feed)

Completion Criteria:
- Staff/Class Rep can create & edit drafts
- Students see only published posts
- Feed endpoint respects roles & audience

Dependencies:
- Milestone 2 (RBAC)
- Milestone 1 (working backend container)

No due date
66% Completed