feat: add multi-step user onboarding wizard

- 5-step wizard: Welcome → Profile → Plan → Storage → All Set!
- New migration 017: onboarding_completed, contact_email, preferred_destination fields
- REST API at /api/onboarding/{status,profile,plan,storage,complete}
- GET /onboarding view with configured-destinations helper
- OAuth callback redirects first-time users to onboarding
- 16 unit tests for all endpoints; 65 total tests pass

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_onboarding.py

@@ -278,7 +278,9 @@ class TestOnboardingAPI:
         _make_profile(ob_session, _TEST_USER["sub"])
         resp = ob_client_authed.post("/api/onboarding/complete")
         assert resp.status_code == 200
-        assert resp.json() == {"success": True}
+        data = resp.json()
+        assert data["success"] is True
+        assert "redirect_url" in data
 
         # Re-query to see persisted value
         ob_session.expire_all()