[Tests] PostEndpoints — add edge-case and filtering tests

[fboucheros]

What to build

PostEndpointsTests.cs covers basic CRUD but is missing edge-case and filtering scenarios.

Acceptance criteria

• Test: GET /api/posts/{id} with a non-existent ID returns 404
• Test: DELETE /api/posts/{id} with a non-existent ID is handled gracefully
• Test: GET /api/posts/ (unread) when all posts are already read returns empty list
• Test: GET /api/posts/read when no posts have been read returns empty list
• Test: POST /api/posts/ saving a post that already exists (duplicate) behaves correctly
• All new tests pass in CI

Blocked by

None — can start immediately.

Priority

🟡 Important — filtering edge cases are currently untested