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/app/routers/campaigns.py

@@ -785,36 +785,18 @@ def remove_test_from_campaign(
 def activate_campaign(
     # Entry: campaign_id
     campaign_id: str,
-    force: bool = Query(False, description="Activate even if start_date is in the future"),
     db: Session = Depends(get_db),
     # admin passes automatically via require_any_role's built-in bypass — do not add "admin" here
     current_user: User = Depends(require_any_role("admin")),
 ):
-    """Activate a campaign, moving it from draft to active.
+    """Admin-only emergency override: activate a draft campaign directly, bypassing the approval queue.
 
-    If the campaign has a start_date in the future and force=False, returns a 409
-    with a warning so the frontend can show a confirmation modal. If force=True,
-    activates immediately regardless of start_date.
+    Every other path to 'active' goes through /submit -> /approve, where the
+    manager sets start_date. A draft campaign never has a start_date set
+    (only /approve sets it, in the same call that moves the campaign to
+    'active'), so there is no "scheduled for the future" case to guard
+    against here anymore.
     """
-    from fastapi import HTTPException
-    campaign_obj = db.query(Campaign).filter(Campaign.id == uuid.UUID(campaign_id)).first()
-    if campaign_obj and campaign_obj.start_date and not force:
-        now = datetime.utcnow()
-        if campaign_obj.start_date > now:
-            raise HTTPException(
-                status_code=409,
-                detail={
-                    "code": "start_date_in_future",
-                    "start_date": campaign_obj.start_date.strftime("%Y-%m-%d"),
-                    "message": (
-                        f"This campaign is scheduled to start on "
-                        f"{campaign_obj.start_date.strftime('%d %b %Y')}. "
-                        f"It will activate automatically on that date. "
-                        f"Do you want to activate it now anyway?"
-                    ),
-                },
-            )
-
     with UnitOfWork(db) as uow:
         # Assign campaign = crud_activate(db, campaign_id)
         campaign = crud_activate(db, campaign_id)