refactor(campaigns): remove dead future-start_date scheduling code

Now that only the manager sets start_date, and only at the moment of
approval (which immediately activates the campaign), a draft campaign
can never have a start_date. This made three things permanently
unreachable: the hourly _run_scheduled_campaign_activation cron job,
the activate_campaign 409/force future-date guard, and a query filter
hiding tests from scheduled-but-inactive draft campaigns. Removes all
three rather than leaving dead code behind. Also adds the missing
manager-cannot-directly-activate router test.

backend/tests/test_campaign_approval_router.py

@@ -279,6 +279,13 @@ def test_admin_can_still_directly_activate_as_override(api, db, red_lead_user, r
     assert resp.json()["status"] == "active"
 
 
+def test_manager_cannot_directly_activate_draft_campaign(api, db, red_lead_user, manager_headers):
+    """A manager approves via /approve — the direct /activate bypass is admin-only."""
+    campaign = _make_draft_campaign(db, red_lead_user.id)
+    resp = api("post", f"/api/v1/campaigns/{campaign.id}/activate", manager_headers)
+    assert resp.status_code == 403
+
+
 def test_create_campaign_payload_has_no_start_date_field(api, db, red_lead_headers):
     resp = api(
         "post",