[Good First Issue] Add pytest tests for milestone CRUD endpoints

[jaytoone]

What needs to be done

Add end-to-end tests for the core milestone CRUD endpoints in server.py.

Currently there is no test suite. We need at least:

• POST /api/northstar/{proj}/milestones — create stone, verify default status=queued
• PATCH /api/northstar/{proj}/milestones/{mid} — status update
• DELETE /api/northstar/{proj}/milestones/{mid} — verify removal

Use pytest + httpx.AsyncClient (already in the dependency graph via FastAPI test client).

Why it matters

Every refactor is currently manual-tested. A basic test suite will catch regressions before they reach production (where this runs live 24/7).

Acceptance criteria

• hub/tests/test_milestones.py exists and passes python -m pytest hub/tests/
• Tests use an in-memory SQLite db (not production ns-events.db)
• At least 3 CRUD test cases covered
• CI-friendly (no tmux, no external services needed)

Estimated effort

2-4 hours for someone comfortable with FastAPI + pytest.

Helpful context

• Main server: hub/hub/server.py
• Endpoint prefix: /api/northstar/{proj}/milestones
• See ARCHITECTURE.md for data model overview
• Existing stub dir: hub/tests/ (currently empty)