[PBI] Notification read/unread state and per-user lifecycle

[cjlapao]

Description

Ensure that the is_read flag is correctly initialised, propagated, and updated across the full lifecycle of a notification — from creation through WebSocket delivery to REST state updates — and that the unread count is always consistent with actual row state.

User Story

As a user, I want to see a reliable count of notifications I haven't read yet, and I want that count to update immediately when I mark notifications as read, so that I always have an accurate picture of what still needs my attention.

Acceptance Criteria

• Default state: All newly created notification rows have is_read = false.
• WS payload includes flag: The notification.created WebSocket message includes is_read: false so the UI can immediately increment its local unread count without a separate API call.
• Mark-read idempotent: Calling PATCH /notifications/:id/read on an already-read notification returns 200 with no error.
• Bulk mark-read atomic: PATCH /notifications/read-all executes as a single UPDATE query. It must not leave partial state if interrupted.
• Unread count accuracy: GET /notifications/unread-count reflects the true database state, not a cached value, so it is always consistent after any read/delete operation.
• Concurrent update safety: Two simultaneous mark-read calls for the same notification do not produce an error or inconsistent state.
• Unit tests: Tests cover initial state, idempotent mark-read, bulk mark-read, count accuracy after each mutation, and concurrent update behaviour.

Definition of Done

• Code implemented following best practices.
• Unit tests written and passing.
• Code reviewed and approved.
• Merged into the main branch.
• Documentation updated (if applicable).
• Deployed to staging/production environment.

Assumptions and Constraints

• Assumption: Database-level consistency is sufficient — no in-memory cache of unread counts is needed at this stage.
• Constraint: The read-all operation must be bounded to the authenticated user's rows only.

Dependencies

No response

Additional Notes

No response