Fix/scope user query by team

[stianjsu]

Scopes the user query in invite_multiple_if_needed by team_id so the fairness algorithm only considers users from the event's own organization.

Key additions

• slack_user_service.get() now receives team_id=event['team_id'] — prevents the lookback window and employee count from being inflated by users from other orgs
• test_multi_tenant_fairness_isolation.py — seeds two orgs (Org A with 6 users, Org B with 12), builds up 6 finalized past events with attendance history, and proves the correct users get invited. Without the fix, the inflated lookback flips the outcome entirely (User1/2/3 invited instead of User4/5/6)

Motivation and Context

In multi-org deployments, slack_user_service.get() was called without team_id, returning all users across every organization. This inflated total_number_of_employees, which controls number_of_events_regarded = ceil(total / per_event) — the lookback window for the fairness algorithm. A larger window pulls in older history and skews invitation priority ordering. No cross-org invitations occurred (the group subquery correctly scopes users), but who gets invited within an org could be wrong.

How Has This Been Tested?

• test_invite_uses_org_scoped_user_count — with correct per-org lookback (ceil(6/3) = 2), User4/5/6 have 0 recent attendances and get invited. With the buggy global count (ceil(18/3) = 6), the window expands, User4/5/6 show 4 attendances each, and User1/2/3 get invited instead. Verified the test fails without the fix and passes with it.
• Full backend suite passes (131 tests)